Loss of upload speed puzzle

[tehidyman]

Since having fibre installed I have run a TBB speed test at least once a day covering 95 days.  My download speed has consistently recorded 37Mbps with very little variation (plus or minus about 0.5Mbps) and upload speed around 9.0Mbps with again little variation until 3 days ago when the upload speed dropped to between 2.9 and 6.8 Mbps and has not recovered.  This lower upload speed is repeated on BT speed tests and others. Download speed unchanged.
Firstly I repeated the test using my old XP machine and confirmed that the upload speed on this machine was still about 9.0 Mbps.  Current machine is Win 7.
I have checked that at start-up only the three regular programs are listed and all I.E and scheduled tasks are disabled and then restarted and retested. No change.
I have disconnected and reconnected to the internet. No change
I cleared out junk files, used disk clean up and defragged and optimised- all gave no change
I have used system restore to restore the computer to an earlier time before the loss of upload speed. No change.
The TBB upload graph now is quite erratic whilst on the XP machine it climbs rapidly and stays steady as it did initially on the Win 7 machine.
Recent test.   http://www.thinkbroadband.com/speedtest/button/137753479354125654253.png

Anything else I could try?

Have removed  malware, virus scanned all clear- no change.
Tested using Win 7 netbook usual 9 Mbps up

[Steve]

If it's just one machine you could try swapping network cables which may have already done or network cards. To test the OS try a Linux live CD which should tell you whether is the Windows OS or a hardware issue.

[tehidyman]

If it's just one machine you could try swapping network cables which may have already done or network cards. To test the OS try a Linux live CD which should tell you whether is the Windows OS or a hardware issue.

Just swapped cables. Took the cable out of the recommended GigE port (where it has been since set up) and plugged into a spare port and upload speed and consistency is perfect.  Put it back in the GigE port and the fault is there again - looks like a problem in the router. Thanks for that idea.

[andrue]

Do you have an Adblocker installed? The one I used to use reduced upload throughput by 50%.

[tehidyman]

Thanks for that suggestion. I tested using Adblock enabled and disabled in both ports and found the adblock made absolutely no difference to the upload speed.  I have concluded that I must have a minor fault at the GigE port.  As I do not get the speed at which that port should be of value I will stick to the other three.