So I'm having an interesting problem. My Mac Pro 3,1 is not seeing all 8 sticks of 2GB RAM. DDR2-667 ECC, etc. All sticks are identical. If all sticks are installed, or if the sticks are in just about any combination, the machine only sees 4GB. If the RAM is installed in slot A, C, A, and C (in both risers, respectively), the machine sees all 8 GB. Every sticks is identical, and every stick works as long as it is in one of these combinations. I've reset the PRAM, SMC, run Memtest, checked the diagnostic LEDs on the logic board and the RAM risers, installed RAM in just about every configuration possible, and this is always the case. Anyone have any ideas?