Sudden drop in fibre speed

[#1]

When the engineer called round a couple of months ago to install FTTC, the line sync speed came up 39M plus odd on his gizmo. The actual functioning line sync was about 35M wired and 25M wireless. This was a huge step up and I was well pleased!
Hpwever, in the last few days speeds have been syncing around 7M to 8M both wired and wireless. I can't see anything particularly wrong anywhere e.g service status or usage. I've tried rebooting modem and router but to no avail. I'm a bit worried I'm actually making the speed worse by rebooting so much.
Any ideas on what might be the issue?
I'm on the LLANISHEN exchange if that helps.
Kev

[#2]

With the normal BT modem you can't check your actual sync speed are your figures actually download speeds?
What does the BT speedtester say your profile is?
This will give an isnight as to whether the problem is reducing sync or perhaps congestion or something else.

[#3]

The sync speed of 39M + was given by the BT engineer when he installed the fibre.
The BT speedtester says my profile is 38717 or similar. Interestingly, when I ran the second test using the default btspeedtester sign-in as opposed to my own plusnet one, I got my normal download speeds back. I've just raised a fault with Plusnet as their own diagnostics suggest something is up. When I tried bypassing the router and connected straight via the modem, I got my normal download speeds again.
Looks like either a profile issue or a hardware issue or both. Either way they should have all the info they need now.
Fingers crossed...

[#4]

Hope you don't mind me crashing in ,but the same thing is happening me. It will be interesting to hear what the outcome is. My gut feeling is we are being capped by plusnet..the bt engineer told me that when plusnet buys bandwidth from bt eg. 400mbps worth @ 40 mbps per customer equals 10 customers.        
But let's say you are being capped to 30mbps per customer. Out of the orginal 400 mbps bought instead of 10 people sharing the speed ,plusnet can now get another 3 people squeezed in .thus making more profit.    Does that make sense. It's what the bt engineer told me  

[#5]

Well there's nothing like spreading a good conspiracy theory is there? Have you read the story of network load during last weekend's Manchester derby game. Capacity to spare too, hardly the marque of an ISP marginal on capacity.

[#6]

im only saying its what the BT man told me and from a business point of view  it would make sense that not everyone needs the full wack every minute of every day but it might explain why everytime we do a speed test its show up shy of what we expect.
however like your stats show ,when the line asks for more bandwidth as in it forces more requests (if that makes sense ) then just maybe some juggling goes on at plusnet and all isp's for that matter and the needs are met.
He said that  BTare pushing out 45mbps  to the cabinet so that we get 40mbps at the  house.
its whenever your isp takes over the line that when see the loss

so like i said "im only saying"

[#7]

Whatever the theories, I can happily report a positive outcome on this one.
Having checked the line, they told me that something had gone awry with the ip profile and it was set much lower than it should have been. It's been reset to 40 MB and I'm a happy bunny 

[#8]

Thought I'd throw my tuppence worth into the conspiracy theory.  From time to time I check my speeds using ThinkBroadband.  I know it's not regarded as the Gold Standard but it's quick and easy.  Today it showed only 12Meg.  The nearest fibre cabinet isn't as close as I'd like and I normally expect to see about 18-20Meg, which is a vast improvement on my previous 1-1.5Meg on Sky copper.  I rebooted the router with no significant improvement.  I ran the BT speed test which confirmed only 12.6Meg with an acceptable range of 20-29Meg.  The IP profile is 29Meg.  I went through to the TAP3 test which showed a download speed of 26.3Meg and a narrative "Your service was found to be performing poorly.  Please raise a fault with your service provider, stating that the BT Performance Tester Tool indicates poor service throughput performance after the second test".  I was about to raise an online support ticket when I decided to run one more test on ThinkBroadband.  Miraculously it showed a speed of 27Meg which I repeated with a similar result.  I don't want to waste the Support guys' time but has anyone any ideas why this happened?  For instance, does the BT Performance Tester automatically notify ISP's of performance issues?  Don't know if it's significant but the IP address for the latest (fast) speed begins with 46, whereas the Ip addresses for the slower speed began with 87 & 91.