cd rom drive disappears & reappears

I've seen other posts about missing CD rom drives, but I think my problem may
be different?
I have Windows Vista basic, on an eMachines PC. I have a DVD rom drive AND a
CD rom drove, both recording drives. Often when I go to access the CDrom
drive, it is missing in My Computer. It is ALSO missing completely in Device
manager. It doesn't seem to matter if there is a CD in the drive at the time
or not. It won't eject either.
USUALLY it reappears if I restart the computer. And there is no exclamation
mark in device manager at any time. And when it does appear, it works
beautifully.
I don't know if it matters, but the two drives are on the same ribbon cable,
with the DVD as the master.
Any helpful suggestions are appreciated!
Amanda (newbie to the forum)