Some USB chipsets are problematic - though its usually the modem that is the root cause of any problems.
The PCI card is an option and a very good one is the Belkin F5U006 4 port model which contains 4 Lucent USB 1.1 controllers which are very tolerant of high current demanding USB ADSL modems.
If this is your answer then shop around - prices vary quite considerably.
But before that: have you got the modem powered off a good over-current handling self-powered hub? How about your MTU being optimised? Maybe you havent removed the power off option on your USB hubs and modem.
Post back if you need more specific info.