my input :its the USB ....... i have used a USB modem for a while and have come across this problem many times....
USB hates sharing IRQ's - try to get it on its own IRQ ....
http://www.usbman.com << -- go there
a good trick is to take all PCI cards out , reboot, then shutdown ..... reboot go into BIOS and disable com2 (serial2) in the bios that will free an IRQ for you ...... then install the USB device.......
after thats installed insert one PCI card in at a time , hopefully the USB controller will get its own IRQ ........
You can share IRQ with a network card that doesnt seem to affect USB .. = PCI slot 3 or 4 by memory