Possible Mobo issue?

[FXL5]

Recently (a couple of weeks) I have been having an issue with my rig where pressing the power button seems to be the equivalent of asking it nicely to boot. About half the time, the fans spin up and lights come on, then it turns itself off, waits, then boots properly. I've tried clearing CMOS, I've reseated the ram, I tried breadboarding it, same issue.

Today, it failed to POST entirely. After a few repeats, I opened it up and cleared the CMOS again. That helped insofar as getting it to give me a BIOS. After setting up and resetting, it booted into Windows. Shutting it down and restarting gave me nothing, then BIOS claiming no HDD, then booted properly into Windows when I exited BIOS (huh?)

However:
The BIOS could not see half the RAM (16GB instead of the 32 installed)
- Yet once in Windows, it all appears to be present and correct.
The pre-bios screen claimed no HDD attached.
- Ditto. All SATA devices are present according to the OS.
The CPU fan header seems to be nonfunctional; it shows an error in the bios, and the fan does nothing even now it's booted.

My PSU voltages check out, there's nothing wrong with the graphics card (swapped with another computer a few days ago to check, same issue on this one, no issue on the other). I haven't tried this today, but the original issue was not alleviated by removing all SATA devices and booting from USB.

I suspect that my motherboard is either failing slowly or has done so in one specific area, but I'd like to solicit opinions from the community.

Rig (minimum):
MSI GTX460
OCZ 700W PSU
Asus P8P67 Pro
Intel i5 2500K
Samsung 500GB HDD
Avexir Blitz 1600MHz 32GB

 

[ShivP]

Try running barebones (CPU, mobo, RAM, PSU, one HDD) with one DIMM at a time and see where that gets you - try each DIMM at a time to see if any are faulty/it helps in any way.

 

[FXL5]

No change, though interestingly on each of those reboots (still all via a forced detour to BIOS) each of the RAM sticks was detected fully. Tried varying the slot as well. No better, but no worse either.

 

[FXL5]

Update: today upon pressing the power button I was treated to fans spinning up, everything turning off, and then the same again. Clearing the CMOS got me as far as the BIOS, after which it attempted to boot into Windows only to stop on the logo before login screen (black screen, blinking screen power light as if PC was off, despite fans etc to the contrary.) Dismantling and breadboarding have returned nothing more than fitful attempts to boot, not even a POST or BIOS that I can tell.

CPU fan still does not spin - definitely the header, as the fan is now connected to a different header so that it can do its job.

RAM checks out (all fine in Memtest on a different PC).

ETA: pulling and reseating the CPU, and then booting from a Linux USB, got it to boot correctly (albeit still via a detour through the BIOS). Still won't boot from SATA, though, regardless of port or HDD.