I think Im having similar problems at times. There will be periods when on a freqeuent basic my connection dies but the sync/rdy lights are still on but dwn/up lights are dead. I reboot the modem and all is fine for x amount of time.
It may be my imagination but it seems worse if you upload as well as download via bittorrent (for demos and freeware movies etc) or other p2p software.
I had one evening using BT when it was disconnecting every 10 mins but then I tried stopping the BT program all was fine for the rest of the evening, back to BT and disconnected again.
Course its your typical nice random fault that if they engineers dont find on call out they can charge you for wasting there time. SO I dont bother ringing just put up with it
Heehee just did it again, for what its worth is wasnt an immediate cutoff download speeds lowered over maybe 10 secs and reduced to 0.