You say playback is choppy, too? Are you playing back audio through your computer's soundcard or are you using a softsynth and routing output back through to the Tascam and listening to it through headphones or speakers connected to that?
If you're using your internal soundcard for playback, try disconnecting the Tascam, then playing back audio - any audio, not just stuff you've recorded. If you still have problems with choppy playback, you might want to consider upgrading or at least seriously optimizing your system. Check for extra processes running in the background like antivirus and firewalls, make sure you don't have any spyware going on(use Ad-Aware or Spybot Search & Destroy to get rid of it all!), defrag your hard drive, etc.
If you can play back audio cleanly withouth the Tascam connected but have problems when you plug it in, you've probably got a problem with your USB chipset - it might not be fully compatible with the Tascam unit. See if there are any BIOS updates for your motherboard, although that probably won't help. It might be a better solution to forget about USB interfaces and get a PCI audiocard for recording, like an Audiophile 2496 or something comparable. You'll avoid all the USB-related problems although if you record from a mic you'll have to get something to replace preamp and phantom power supply functions of the Tascam.