Speeds all over the place

[#1]

I'm suddenly getting wildly varying speeds, none of them very good. Couple of questions:
Do I understand correctly that I can't report a fault on my "up to 8Mb" scheme if I'm getting actuals over 400K ? I've done a few BT speedtesters over last few days with some results showing synch speeds of 3500-5200 and corresponding reasonable IP speeds but with actual speeds of around 430k. It doesn't sound right to me, but if that's all I'm entitled to.....
Also, I think I've understood the speed being throttled by the IP speed setting and it taking up to 3 days to reset it if it get low.....but I seem to be getting the odd very low resynch down at 160k which resets the blasted thing to 135k giving actual speed as low as 119k. When I notice and reboot the router its quite likely to synch at 4500+ but of course that does me no good. So the question is why have I suddenly started getting these occasional very low resynchs ? Nothing changed in my setup but it does only seem to be doing it since they've started digging about 100m down the road. Is that likely to be anything to do with it ?
Steve

[#2]

Hi Steve,
If the throughout is showing as OK on the BT Speedtest and not on others, it's often a case of misconfigured MTU & rWin settings.
Have a look at this video:
http://www.plus.net/support/broadband/troubleshooting/rwin_video.shtml

[#3]

Quote from: Jameseh

If the throughout is showing as OK on the BT Speedtest and not on others, it's often a case of misconfigured MTU & rWin settings.

Thanks for that but BT Speedtest and other tests all show actual download speeds of 430-450k except when the IP profile is lower. And rWin and MTU are within about 5% of the recommendations.
I was mainly asking if that's all I can expect to see. E.g. BT Speedtest shows IP Profile and Downstream connection rate much higher e.g. IP Profile 4500, Connection 5248, Actual IP throughput 452 or from another test IP 3000, Connection 3520, Actual 446 (both off-peak so there shouldn't be any contention problems). The examples I see of other people's Speedtest results seem to show actual downstream speed much nearer to the IP Profile rate (or perhaps I'm just not understanding the terminology....networking always confuses me).
Today I've had another 160k connection blip so at the moment I'm stuck with 135k IP profile and so I can understand the current low throughput.  I just don't understand a) why 450k throughput on a 4500k profile/5248k connect is apparently o.k. or b) what's causing the odd very shortlived slow 160k synchs which mess up the IP profile thus making the throughput even lower.
Steve

[#4]

If you're getting a number of connection blips, it's very likely to be the cause of your poor speeds.
I'd recommend trying to use a different filter, ensuring that your router is plugging directly into your master socket via a filter and that you have all active phone sockets filtered.
Trying a different modem/router is also never a bad idea.