Best thing to do is probably to upload all of your information (waypoints, routes, etc) into MapSource (i.e. back them up) then do a hard reset of the unit:
Press PAGE+OK+POWER
and install the latest updates from Garmin:
Garmin: Quest Updates & Downloads
Obviously, this will delete all your data from the Quest so make sure you've got it all copied out first. . . .
It sounds like a software problem, so I'd be surprised if this doen't resolve it.
David