A bad USB/Firewire cable is the culprit more often than you'd think. I've been telling anybody who contacts me with interface problems of any kind to first replace the cable, then try different USB/Firewire ports on your computer. Those 2 things seem to clear up more than half of the really odd problems people have.
_________________________
http://www.youtube.com/user/MRJSTUDIOS

http://www.facebook.com/michaelrjohnsonstudios

Download my custom 11Rack Rig files here:
http://www.mrjguitars.com/mrj11rigs.html