I have had a few MDD that do that. One ended up being the processor and the other a motherboard, worked out well, I just popped the good processor in the good motherboard and we had one good machine! lol.
MacBook Pro, 2.2GHz SR Core 2 Duo, 120GB HD, 2GB RAM,
PowerMac G5, DP 2GHz, 2GB RAM, 1x500GB HD, Sonnet Tempo X4P, 2TB RAID, ATI 9650 256MB.