I have the same problem with my Keystation. What I have figured out is that the USB wire and the erm...port on the keyboard do not match 100%. I mean, sometimes the wire moves too much and *bang* we have lost the input until I restart the unit.
Maybe similar problem? Both are M-Audio products...who knows. I've blamed my problem for faulty construction as the Keystation series IS the cheapest of M-Audio keyboard products.