What am I missing, Intermittant POST fail

HP i7-860, 460w psu, hd5770, 8GB in four 2GB dimms.

The system failed to post. Power off, pulled plug, no change.
Removed video card, BIOS post'd, beeped no video. Cool.
Swap new video card for old, system posted, worked. Cool.
Swapped back in old video card. System posted, worked!. Decided reseating video fixed something. Done.

1 week later system failed to POST again.
Pulled video card. Still failed to Post. Bummer, wasn't video.
Swapped PSU. No change, still failing to post without video installed. Guess its not video or PSU.
Removed memory. POST'ed fine, beeped no memory.
Added 1 of 4 memory DIMMs back in, no post. power on, fans on, disk started. But no video and disk did not continue to seek, so system was not booting w/o video.
Removed dimm, put different DIMM in different slot, POST. Progress, must be bad dimm.
Added back other 2 dimms for total of 3 dimms, POST. cool.
Added back final dimm. POSTed just fine. No problem. WTF.

Net PC currently has new PSU installed, old video, old dimms. Problem leading to post fail never found.

Suggestions ??? I am expecting the PC to lock up again.

re overclock: I forgot to post that. Everything is stock clock, memory timings default set by HP MB bios loose. Even hd5770 is running stock clock.

I'm hoping you're right and it's something like dust in the dimm slot. If it fails again I'm pulling 1/2 the dimms and see how long before the next POST problem then swap the memory.

Any other suggestions on what to do. I don't really want to swap the MB.

hey, long time no see. thx for posting. I didn't see any signs of cmos battery fail (CMOS Read Error, CMOS checksum error, or CMOS Battery Fail message from bios, etc.) but that's not a bad part to replace. It's cheap and easy.

If the PC is plugged in but powered off is the CMOS battery used? Thinking it is not, that standby power holds up the cmos values but not sure why I think that.