CD Drive Problems

I have no illusions that readers here are particularly tech savvy or anything, but maybe somebody has a decent solution to a problem I've been having. After my computer has been turned off for a long period of time (such as overnight), it starts up fine, but within ten or fifteen minutes (or less), it freezes up completely. When I reboot, it'll come back and is mostly fine (it may freeze again in three or four hours, but it may not).

You may notice that I wrote, "mostly fine." I've got a DVD-ROM drive and a CD-RW drive, and they have no problems when the machine boots the first time. But after it freezes and I reboot it, it loses touch with them entirely. The machine completely won't acknowledge that there are the two drives hooked up to it (the DVD came installed, and I put the CD-RW in four or five years ago). If I turn the computer off and restart it, the drives won't reappear. I haven't found how long the thing has to be off before the drives come back, but I also don't know if that's the same amount of time it has to be off so it freezes again a few minutes after I turn it on.

If anybody has any ideas--even just stopgap measures, I'd be very appreciative.