Speeds all over the place over the last week or so

[#1]

I'm getting massive speed drops on my ADSL 2 service over the last week or so, for no apparent reason. Then it just as suddenly comes back. BT Speedtest is typically showing 14 Mbps down/0.76 up when it's good and something ridiculous like 1.7 Mbps down/0.04 Mbps up when it's broken. It had been behaving like this for a while about six months ago, but it's been reasonably reliable for a few months up until now.
I couldn't be bothered going through the 18-step "Derp is it plugged in?" troubleshooting wizard PlusNet hides the support ticket system behind to raise a ticket each time—the fact that it spontaneously fixes itself indicates the problem isn't on my end.
Is there anything else I can do? Can any of the numbers (Noise margin, Line attenuation...) the modem gives me point to where the problem might lie?
Cheers, Robert.

[#2]

Hi Robert,
A warm welcome to the forums.
First I need to 'reel you in' - the purpose of the '18 step trouble-shooter' is multi-fold: there is a spectrum of ISSUES which can cause the SYMPTOMS you have reported, from your WiFi, internal phone wiring (coupled with RFI noise), your phone line, the exchange's capacity.  Any number of event based or time based issues can come and go, so that there appears to be a 'spontaneous' fix.
Finally that multi-step trouble shooter helps to avoid the risk of calling out a BTOR engineer and he determines that the fault is with something your side of the master socket, which will attract a hefty call out charge from BTOR!
Yes those figures might help point you in the right direction, but not those figures alone.
Please look at the speed issues thread below and post back here ALL of the diagnostic information requested.
Do a quite line test - dial 17070 option 2 ideally using a corded handset.  Describe how your router is connected to the phone line.
Is there any time pattern to the slow speed?

[#3]

Hello there. Sorry that our broadband troubleshooter is quite lengthy. I'll outline the required checks here for speed faults.
I'd recommend testing your speed over a wired connection (if you haven't already). If the speed is still slow, try plugging the router into your test socket (found by unscrewing the faceplate of your master phone socket) and then retest the speed. You may need to wait 24 hours for our connection profile to adjust to any new speed improvements with the connection.
If the problem still exists and you have a Plusnet router less than 12 months old, then you can safely raise a fault by going to http://faults.plus.net as these checks I've outlined are designed to rule out the BT engineer charges that Townman mentioned.
N.B. If your router is more than 12 months old, or a third party one, I'd recommend first trying a different router as it wouldn't be covered under warranty if the engineer determines the fault is the router.
Finally, if you discover that your speed is OK over a wired connection, then this guide should help your wireless connectivity
http://www.plus.net/support/broadband/wireless_broadband/wireless_signal.shtml
I hope this helps