Disappearing CD Drive

I acquired an older Dell PC with a P3, 1 GHz processor. The CDRom kept disappearing. Sometimes it would be in “My Computer”, and sometimes it wouldn’t. I considered that it may be a BIOS problem and since the PC’s warrantee had expired, Dell would be of no assistance. So, I bought a new board. I transferred everything to the new board and fired the PC up. Everything works fine, except the CDRom still disappears from time to time; usually at the most inopportune time.
What may be causing this and how do I overcome this situation?