Sync speed AND noise margin dropped!

[#1]

Hi,
I’ve been synced at 4768k for a while.  My noise margin sits at around 11 db all day till about 4 pm when it drops a fair bit – and stays pretty low ‘till the following morning.
This morning it was at around 11.6 so I used DMT to see if I could force a higher sync speed.  After DMT had gone thru’ the motions I find that my noise margin has dropped to 10.9 – but my sync speed has also dropped - to 4704! 
Is this normal?  I sort of assumed that with a drop in the noise margin I could get my sync speed up a bit.  It has been up to well over 5000k at one time but admittedly my connection was a bit unstable then but I did feel that I could perhaps get it up to ~5000k safely.
(maybe a lesson to be learnt here – leave well alone when things appear to be working)

[#2]

Hiya,
Were there an increased number of errors on your line with the slightly lower SNR?
It's possible that if there were, this could have caused a drop in your sync speed, whereas with the higher SNR, there were fewer errors, causing a slightly higher sync speed.

[#3]

JamesH - thanks for that.
I wasn't checking errors, maybe I should have - before and after - as you say that might have explained it.
Bound to say tho' that I'm downloading at anything up to 3.9 megs with an IP profile of 4000 on what appears to be a longish and noisy line so I'm well chuffed and it's thanks to the PN guys.
Hope to get my daughter signed up as well but at the moment her line is 'ceased' and I'm waiting for Mandy to come back to me - and tell me the line can be 'unceased' for less than the £124 or whatever BT want.

[#4]

looking at the errors on the dmt capture on the other post looks like you could be trying to push your line abit to much.
I game using an xbox so have error checking off at exchange(interleaving) to get better ping times and find my line is stable with a line profile of 2.5mb-3mb any higher with interleaving switched off the errors on the line out weigh having more bandwidth.
As you have interleaving on you can push your line abit more but still theres a cut off point.
you will notice on dmt thing called interleaving depth and on the other post your synced at 4758 and interleaving depth is running at 32.
bein awhile since i was switched to fast path but i think 32 means its working hard to correct errors.
syncing lower will decrease the interleaving depth.
im sure mine use to run at 8 or 16 when my interleaved path was stable as rock.
http://community.plus.net/forum/index.php/topic,71041.0.html

[#5]

ASBO DOG - thanks for that.
> looks like you could be trying to push your line abit to much <
Yes I think that's the long and short of it...
Nice to try and squeeze the last bit of performance out of the system but realistically I suppose you reach a point where it's best left alone...
Thanks again