I have spent the past week or so tracking down an issue where my Mac Pro 1,1 would reboot out of the blue. No rhyme nor reason to it. Finally decided that the issue was probably RAM. MemTest crashed about halfway through the first cycle. Tested just upper Riser with RAM removed from the bottom riser. MemTest ran clean. Moved lower board to top, put RAM back in, moved top board to bottom with RAM removed. Again MemTest ran clean. Finally restored original configuration and MemTest again crashed partway through the block test. Concluded that the lower riser slot on the mother board is bad, or there is a mother board problem that only effects the lower DIMM riser. Currently running on half RAM with the lower board empty. Quite stable in this configuration. Two questions: Would it be better to remove the lower board entirely? And can I fill the secondary pair of slots on the upper board, when the lower board is empty or removed? For legacy reasons I would like to keep this old guy running a few more years.