Been there!
Pipex's Indian customer support weren't much help, but the UK ones were a lot better. That said, I'm still having a problem that they can't diagnose - line drops, where the ADSL won't reconnect to Pipex's computer.
In your case: I had similar problems with a Netgear 834G. I found in the end that it had toggled itself from PPPoA to PPPoE. Changing it back, and setting VC/MUX, VPI/VCI to 0/38, and switching between PAP and CHAP eventually sorted it. In the end, other problems with that router led to the purchase of a Draytek which has been fine.
I can't remember the other settings for the Netgear, but I remember playing with what seemed like an infinity of permutations and combinations before hitting the right one.