Try getting reference drivers from the chipset manufacturer instead of from the company that produced the modem. For example if you have a PCTEL chipset check the PCTEL website for drivers.
If the firmware is upgradable make sure you have the latest version.
Try installing NT4 Drivers instead.
If none of these solve the problem and you are sure it's your modem that's causing the problem, treat yourself to a new modem...they are dirt cheap.