Powerline adaptors - mixed results - advice welcomed

[#1]

I hope I've chosen the right forum for this question....
After finding that my phone extension point was slowing down my download speed signficantly, I decided to change my approach. So rather than plugging my router into the extension socket  and the pc direct into the router, I've moved the router so that it plugs into the master socket elsewhere in the house and am using 200 Mbps devolo powerline adapters to connect to my pc.
I then noticed that the download speed on the pc had become really slow. Using broadbandspeedchecker from the pc produced results of 0.5Mbps, whereas my laptop was giving me 8 to 9Mbps when connected wirelessly. I thought it must be due to the adapters, so I unplugged the ethernet cable from the pc and put it in the laptop. Running the speed check on the laptop then produced a result of 9 to 10 Mbps. I then reconnected the pc to the powerline and got the same poor performance. Today I've left the pc on all day and have been automatically measuring the download speed every hour (using jdast software). The result was the same all day at less than 0.5Mbps until late this evening when it suddenly went up to 2. I connected the laptop to the same adapter and that measured the same high speeds as before.
Does anyone know why the pc would be so slow when connected via the powerline compared to the laptop?
Thanks

[#2]

What results do you get from the Devolo Cockpit software ?
http://www.devolo.com/consumer/dlan-cockpit-most-simple-configuration-software-for-your-dlan-network...

Have you updated ALL the plugs to the latest firmware (v4.4.0.5) ?
http://www.devolo.com/downloads/software/firmware-dlan-200-av-windows-4-4-0-5.exe

Try swapping the positions of the Devolo plugs.

Are all the LEDs on the Devolo plugs green ?, or are any showing amber or red ?

[#3]

I have seen a faulty PC power supply that injected huge amounts of high frequency noise onto the mains.  Was your PC still on when you tested the laptop?

[#4]

I know it's a pain, but do some testing with the powerline adapters at different places. I presume you've only got 2, but if you have more then disconnect all but 2.  Leave one with the router plugged in, then put the other one into the closest power socket and try a speed test.  Move them further apart and see what happens.  Try the second plug in different sockets around the house.  You may find this approach identifies quirks in your wiring.
I have 4 Homeplug 200's with one next to the router for the same reason as you, and I only get full speed (32mbps - I'm on fibre) when speedtesting on a laptop plugged directly into the router.  Elsewhere in the house the speed never goes above 20mbps, and drops to about 15mbps upstairs.  However 6 months ago I was on 2mpbs ADSL so I really don't mind the slight loss.
-Dave

[#5]

One possible cause might be a failure of so-called 'auto-negotiation' between the powerline adapter and the PC. In particular, auto-negotiation is used as a mechanism by both ends of an Ethernet link to determine performance parameters (speed and duplex) of each other.
If this negotiation fails - perhaps one side doesn't support it, is buggy or has simply been manually configured not to negotiate, then the auto-negotiation and fail. A common failure mode is that one side thinks full duplex is in operation whereas the other can only support half duplex. The end result for the latter device is that it detects collisions if it tries to speak at the same time as the full duplex device (which can talk and listen at the same time). Mass confusion then ensues and whilst the connection still works it'll be dog slow.
You haven't said what OS you're using, but assuming it might be Windows I'm not familiar with where the setting is made/verified/observed on it however it is likely somewhere in the 'Properties' of the network adapter under Control Panel. Someone else might be able to direct further. Depending on the current setting (it should be auto) it might be worthwhile forcing 100Mbps full-duplex on the PC if you can do likewise on the adapter also. Contrary to what you might expect, it won't work if you set it at fixed at one end and auto on the other - the negotiation protocol is not a detection based mechanism but rather an active dialogue between both sides. Is it a 100Mbps adapter in the PC?
Given your tests with the laptop show that the powerline adapters are capable of much higher data rates in your environment it certainly points towards an issue with the PC rather than the adapters (of course there could be circumstances within which this is not fully conclusive).
Mathew

[#6]

Thanks for these replies. I've tried switching the powerline adapters around, etc and no difference. I've still got the same fundamental issue. Adapter plugged in from which cable to pc results in 2.3Mbps. Unplug cable and plug it into laptop results in 9-10 Mbps. 
I've looked for differences in the settings of the laptop and pc and wonder if the problem is related to what MJN's suggested in the last post. The laptop's 'network connection settings' in XP control panel show a connection speed of 100 Mbps, but on the PC this is 10Mbps. Looking further, the laptop's 'Network adapter Link speed and duplex' setting is set to auto detect, but on the PC I can't successfully select that mode - all I can go for is 10 Mbps half duplex. Any of the of the other options just drops the connection altogether.  Could this be causing the difference in performance and, if so, how do I change the setting on the PC?
Thanks in anticipation.....

[#7]

Another possibility could be a problem with the network card in the PC. Is the driver up to date, firmware?
Have you ever been able to see full 100mb connection on the PC using just a normal switch or some other device?

[#8]

Quote from: Keymer

Looking further, the laptop's 'Network adapter Link speed and duplex' setting is set to auto detect, but on the PC I can't successfully select that mode - all I can go for is 10 Mbps half duplex. Any of the of the other options just drops the connection altogether.  Could this be causing the difference in performance and, if so, how do I change the setting on the PC?

Is it definitely a 100Mbps adapter? What make/model is it? (Even if built into the motherboard it'll still be based on a particular chipset just like a dedicated card)
Mathew

[#9]

Since it works fine with the laptop, it's not a problem with the network.  I'd suspect the PC network card.  It may be that the driver Windows has selected for the NIC isn't quite the right one  I've often found that the manufacturer's driver works better than the Microsoft one that Windows picks., and sometimes Windows just plain gets it wrong.
What make and model of card is it?  Try going to the manufacturer's web site and hunting for it. Even if it's an old card you'll probably find an XP-compatible driver. For example the 3Com series of 905 cards are incredibly old, but you can still download 3Com's own drivers for them.
I did have something very similar myself last year, but mine was caused by a faulty network switch box, and I'm guessing you don't have one of those.
-Dave