CD-Rom/DVD Drives disabled

Something is causing my DVD and CD-Rom drives to disable themself's as my
computer is running. The activity light on the DVD drive blinks all the time
and I can't see the drive when I goto My Computer.

I've shutdown the system and rebooted but that only works 3 out of 10
times. Even restoring the system to an earlier date works for a short time.
Is there a way to enable the drives without shutting down ?

Thanks
Dick