Okay, the system first:
Gigabyte EP45-DS3R mobo
Intel Core2 Duo E8500
Corsair DDR2 (800mhz) 2x2048
Original symptoms - random freezing in games, such as Civ5, Starcraft2, Rift, Borderlands, etc. Might go weeks without a freeze, then get 3 in one night. CPU and GPU temps don't get absurdly high - maybe high 50's (Celsius). Freezing locks everything up - can only do a manual reboot at that point, so unable to see final temps. System has been running for 3 years - only started seeing this behavior in the past year.
Running memtest86+ (v4.2), it completely freezes at about 4:47 in the first pass. Happens every time.
Pulling the stick from slot 2, memtest runs fine - ran several passes
Swapping the sticks in slot 0, memtest runs fine - ran several passes
Putting the stick back in slow 2 (sticks are now swapped in place), memtest freezes at 4:47-ish
So, this seems to point to the slot or the controller? Is there a program that will run diags on this to confirm? I don't mind putting out for a new mobo and cpu, or new memory - just don't want to do so if it's something completely different.