Inconsistent bootup behavior M3A32-MVP

[Kat1100]

Greetings everybody,

Since a while, I have some strange instability isues with a system, based on the Asus M3A32-MVP Deluxe.
The system ran fine for about 2 years, after initial problems due to the MB's incompatibility with the Corsair PSU I initially used.

At some point, the system started to show quite random instability after cold booting at times. memory errors, write errors on the HDDs, driver instability, - all very random.
I started to look after setup settings, trying to nail down the problem, which in many cases lead to a a system state, that would be stable in all known burn in tests. However, after some time, the problems always reaccured.
What I found out recently is the fact, that the system is absolutely stable if I just boot it into it's bios setup, save it once (no matter if I actually did any change or not) and do the rest of the boot sequence. In this case, the system is absolutely stable until the next cold boot, no matter how long it will run or how many warm boots are done in between.

Now I'm a bit puzzled as to where the problem might acutally be. At least I'm guessing it has to be the MB, as I can really rule out a PSU problem (it's a 750W PCP&C, that is absolutely ok for this system setup and rather new).
What do you think might be something to look into? - MB battery? Damaged Condis on the MB?
I already tried flashing the BIOS with 2 different recent versions. Any OCing or the like also doesn't seem to be responsible, as the behavior is exactly the same in stock setting.

Some more system info:

CPU: AMD Phenom II 940
GPU: ATI 3870 x2 (still subject to change, but on't touch it until this error is nailed)
RAM: 4x 2GB OCZ Platinum 1066 5-5-5-18 (@ 800 5-5-5-15 as a failsafe mode)
HDD: 2x WD 500 GB in RAID 0 as main array, a single 500 GB for data
PSU: PCP&C 750W Crossfire edition
Cooling: efficiently cooled NZXT case (8 case fans), Zalman CNPS 9500 on CPU, G.E.I.L. cooling unit on RAM

Any help is more than welcome. In 15 years of building systems I didn't run into such a stubborn box.

 

[pjmelect]

I wouldn't rule out a faulty power supply just yet, at least try to borrow another one to see if it cures the problem. If the problem is not the power supply then it is almost certainly a faulty motherboard Do you see any bulging capacitor tops on the motherboard?

 

[Kat1100]

Hmm, - I'll try if I can get anotehr PSU. However, It's the second brand PSU in the system already, so I really doubted it to be the issue here.
Most likely it's the MB I think, though the capacitors all look good, just as the soldering (I just had the box opened up once again today to do some more optical check up). What's really strange is this sort of behavior where you need to save the bios settings once after boot to get a stable system. Booting into BIOS without saving leads to reproducably instable results, just as reboots after a boot without going into BIOS and saving it. However, just as I said, it doesn't matter if I actually do any changes before saving, it's the saving that does the job.

 

[pjmelect]

Have you tried clearing the CMOS memory?

 

[Kat1100]

Yes, each time I did any BIOS update, I also did a full reset of the CMOS memory (power disconnect, take battery our of MB, set reset jumper for 10 secs and back).
In fact last time I did this was yesterday, as I saw there was another new BIOS version. The outcome is the same as usual unfortunately