Friend of mine had the same problem with a GPS and a generic USB-to-Serial adaptor (he did A/V installs - all the programmable stuff uses native serial for some idiotic reason) where one would occasionally drop out. For him it was always the adaptor.
The solution was to boot with only one (or neither) plugged in, and then plug it in later. Dunno if that's possible in your case, but it should work.
Possibly, removing one or the other's driver, unplugging the unit, rebooting, then plugging the unit in and reinstalling the driver MIGHT work. That's what they kept telling my friend to do, anyway.