Throughput problem - any ideas?

I'm 6 weeks into a battle with BT BB to try and solve this - From the beginning of March, my throughput dropped to around 500kbps, but peak time only. Off peak (ie: 11:00 am on a weekday or after midnight) throughput was back to (the more usual for me) 5.5Mbps.
On the 31st March, at 12:35 I ran a speedtest (DSL Zone UK) that reported a download speed of 6397kbps. At 13:01 on the same day, d/l was reported at 240kbps and has been stuck at that ever since.
Last Thursday, BT BB sent a BT engineer to check the line. He could find no fault, and confirmed the line stats (below) and also that there were no errors on the line. He said he would escalate the fault to BTW, who have now referred back to the Line Faults Department, who have arranged another engineer's visit.
Am I missing something here? The line has always, always synced at the maximum rate. Noise Margins are fairly stable, and until last month/last week everything was fine with d/l averaging 3.5Mbps overall. I can never connect to the BT Speedtester site, so I'm not sure what my profile is set at (Actually I can only get to the site if I enter the speedtester log in, but then it won't run the test!), but I suspect it's stuck (again), unless anyone can shed any light on why this may have happened?

Thanks.

Downstream Rate: 8128 kbps
Upstream Rate: 448 kbps
Current Noise Margin: 10.0 dB (Downstream), 25.0 db (Upstream)
Current Attenuation: 21.0 dB (Downstream), 10.0 db (Upstream)
Current Output Power: 19.9 dB (Downstream), 11.9 db (Upstream)

There was some talk of BTBB having some 'issues' at the moment....when the next engineer comes, ask them to run a 'repair woosh test' this will allow them to see your Bras profile, which, if your sync is the stable 8128 as you say, should be 7150Kbps.

Thanks Zarjaz. I'll give that a try!

Engineer just left.
Profile stuck at 288.
I've had this before, when I switched to MAX - For some reason, my profile never increases after a drop - regardless of the fact that I've never seen the router sync at less than full speed (although of course, it must have done at least once!).
Last year it took me six weeks to get the profile raised (it was stuck on 2Mb) - about the same this time.

One thing to say in favour of BT though, Support from India is atrocious - may as well not have it at all tbh, but once an engineer is on the case - different story. Marvellous service and they know what they're talking about!
Just a pity that BT BB / BTW didn't take note of the first engineer's report (who came last week and could find no problem with the line) and just reset the profile, instead of bouncing it back to Line Faults, who in turn re-referred it to the engineers.

In reply to:

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Just a pity that BT BB / BTW didn't take note of the first engineer's report (who came last week and could find no problem with the line) and just reset the profile, instead of bouncing it back to Line Faults, who in turn re-referred it to the engineers.

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Sad to say thats too true. Since the start of A1000 faults via NEO, rather than the old GPA tasks raised by ECO, no one seems to pay any attention at all to what the engineers report states. v.sad.