I purchased a pair of 667MHz for my Mac Pro 1.1 a few years ago. There were no problems until earlier this week my computer began to slow down. I checked the Activity monitor which showed a small sliver of free memory and then checked the System Profiler. The original pair of 512 DIMMs were listed but the 2nd pair of 2GB DIMMS were missing from the report.
I know for a fact that the DIMMS were installed because the panel was open and I had removed the trays and reseated the DIMM sticks before strating up again.
The only symptom of the DIMMs disappearing was the slowdown of my Mac Pro. I tried swapping the position of the pairs, purchased pair to the top tray, original pair to the bottom tray. This resulted in a startup error. I've switched the position back and it's working, albeit a bit slowly.
Clearly my extra DIMMs went bad so I've removed them and have another pair on order from OWC. But I am curious, is there a way to test memory that your computer can't see?