Hiya mand
I assume that there is no problem with negative noise margin for the evening as long as it stays connected?
Believe it or not, this is your problem. When the noise margin is this low, the router struggles to "hear" the signal because there is more interference (noise) than signal on the line.
That means loads of packets are being corrupted so the router has to re-request ones it misses. If it has to request a lot of packets again, your speed will drop considerably because so much data is having to be requested at least twice!
Most routers will give up at this point and re-sync. The DG834 doesn't and tries to hold on for dear life! This may appear to be a good thing, however it can also be a bad thing, as you are finding out. There is another firmware upgrade due soon that should address this issue somewhat.
Unfortunately it looks like you might have to live with the 3.5Mb profile as the higher sync looks to be a little too much for your line to handle