Sunday my Macbook Pro (Core 2 Duo) downloaded a software update and wanted to reboot, so I said "OK." When it started back up, I got three beeps and then the power light flashed. Obviously it didn't boot--it had failed the power on self test with a RAM error. Not good.
I tried reseating the memory, no joy. Finally, I discovered that I could put either SIMM into the top slot and it would boot, but putting anything in the bottom slot failed. So, I booted with just the 2Gb card and made a clone of the machine to another Macbook Pro I had sitting around. Whew!
The next morning I put both cards in with the intent of taking it in for repairs, but it booted fine. I swapped them. It booted fine, but only showed 1Gb of RAM. I rebooted and saw all of memory.
I decided to try the Apple Hardware diagnostics (boot with the OS disk that came with your machine while holding down the "D" key). No problems found. I thought maybe it was a heat issue, so I downloaded something to exercise the CPU and get the heat up. Still, it boots fine.
At this point, I've got a machine that seems to work that I don't trust. Argh! Any ideas?