Great Speeds for 4 Years - Now Rubbish!

[#1]

Having had great service and top-notch speeds from Plusnet for 4 years I'm now suffering badly. Please can anyone give advice?
This is the problem:-
Until about 4 days ago my download speeds have been consistently high (4000 - 6000 kbps) regardless of time of day. Now I'm seeing a consistent pattern with 'busy time' downloads of just 500 kbps and 'quiet time' downloads still at an acceptable 4000kbps. Like most other people I want a good service in the evenings, but 500 is pretty useless, even for just browsing.
This is what I've tried or not tried:-
1. Numerous speed  checks at different times (Plusnet, ADSL Guide and BT) - hence pattern established as above. Exchange is Green.
2. Changed filter - no improvement.
3. AVG, Adaware, Spybot scans all clear.
4. Don't have another router to try.
5. No hardware or software changes have been done in last few months.
6. Router is plugged straight into BT socket, as it always has been.
7. Problem is on both PC's in the house which are on wired network.
I would say with reasonable confidence that the problem isn't on my side of the BT socket - because I get good speeds at off-peak times.
I'm on the old Plusnet PAYG which I thought gives good speeds - so I'm left wondering whether Plusnet have recently downgraded the performance of PAYG. Or did a lot of people get broadband for Xmas and now the BT exchange can't cope.
Any suggestions welcome.

[#2]

Also, check your exchange at the Exchange Checker.  It may be that the exchange is currently contended and can give you information as to when BT hope to have this cleared by.
B.

[#3]

Thanks Barry & PJ, have done as you suggested.
BT Speed Test Results (Sat 17:30)
    IP profile for your line is - 7150 kbps
    DSL connection rate: 448 kbps(UP-STREAM)  8128 kbps(DOWN-STREAM)
    Actual IP throughput achieved during the test was - 1012 kbps
The actual IP throughput looks a bit better than same time on Friday which was 500 ish. Browsing performance is still very bad though, and Plusnet speed checker shows between 600 and 1000 this Sat evening.
Will run BT speed checker again on Sunday morning for comparison.
Have carried out Exchange Checker and there are no reported problems.
Also, I do not have have any usage limits.

[#4]

PJ,
Thanks for moving it to the correct place.
Is this what you were referring to:-
Your current broadband speed is 8000 Kbps

[#5]

Thanks PJ,
Will do exactly as you say and will continue monitoring speeds over the next few days to check the difference between peak and off-peak times.
This Sunday morning, speeds were excellent, up to 6600kbps with occasional drops to 4000. (BT Speedchecker and Mybroadbandspeed.co.uk)
Will now reboot router as recommended, although this has been done several times over the last few days. Will give it a few more days before raising fault, in case things improve.

[#6]

Hi tfmason,
I've had a quick look at your connection and I'm unable to see anything untoward that would be causing you to siffer from slow speeds.  also from reading your forum post, I think you've probably tried most things that you can realistically do (barring moving house :))
I'd be inclined to think that your problem is a congestion one at your local exchange and as such would be interested to see how much your speeds differ during off peak times.  It's not going to be an issue with our network as we would have seen numerous other complaints like yours.
Let us know how you get on.

[#7]

Having become very keen to establish whether it's just me who is suffering with very poor download speeds at peak times on my exchange, I devised a little test in conjunction with my neighbour (who is on a BT package) to see who gets the worst speeds at peak times.
By communicating via our mobiles to start each test run and ensuring there was no other traffic other than the test download, we carried out a synchronised back to back test using http://www.mybroadbandspeed.co.uk/ ; each pressing the button at precisely the same time. Both are wired networks.
The results are shown below. I think you will agree its just about a draw.
Time BT kbps Plusnet kbps
19:32 1587 855
19:34 1102 606
19:35 531 547
19:36 430 1362
Minimum 430 547
Maximum 1587 1362
Average 913 843
IP Profile 6500 7150
Approx
Distance to Exchange
   400m    400m
If there is nothing wrong with the exchange, as indicated by BT, it leaves me wondering how bad the speeds will get as more and more people start using Broadband.
I would be very keen to know for comparison, what download speeds other people throughout the country are experiencing at peak times (e.g. 19:30 hrs)? Perhaps a few people could reply, indicating whether it's on a wired or wireless network.
Incidentally, I often get 6000kbps off peak, which I would say is not too bad for an 8Mb speed package.