Cold boot problems

I have system stability problems for a long time. I always thought it was my TV tuner card, but now that I have it removed, the problems still persist.

Problem:
The system can run fine for some days (in which I boot up every morning), and suddenly be heaviliy instable on some days: black screen during bios boot, Windows crashes or reboots during boot, or the blue bios setup freezes, or a reboot in the blue bios setup. During these problems, I have to remove power from the PC a number of times, wait, try again etc. Or simply leave it off for some hours and then try it again. The problems seem to be called "cold boot problems" in by some users in similar cases.

Specs:
-CPU: AMD Phenom II X4 955 @ 3.20 GHz (normal speed) (temperature around 42 degrees celsius)
-RAM: 4 GB total, Kingston KVR1333D3N9K2/4G (kit of 2 modules)
-MoBo: Asus M4A79XTD EVO (Chipset: AMD 790X / SB750)
-GPU: Club3D HD 5750 1GB GDDR5 PCI-E (passive cooled by default) (ATI Radeon HD 5750 chipset)
-PSU: Seasonic M12 II Bronze 620W (previous PSU: Seasonic S12-430, 430W had similar problems, if I'm correct)
-OS: Windows 7 64bit
-BIOS: set to default, except: serial port disabled, firewire disabled, and AHCI mode instead of IDE mode.

Memtest runs without problems for at least 8 hours during a test. Maybe it won't run normal when the system is instable, although a quick test using Win7 memtest, didn't show any problems on a single run.
I replaced hard disks, so that should also not be the problem. Also tried different sata and power cables. Also reinstalled Windows numerous times. It shouldn't be Windows related, because the BIOS setup itself is even instable during instability.

The memory modules that I have are not explicitly mentioned in the Asus memory QVL, although a similar module is: KVR1333D3N9/1G, which is probably a single 1GB module. So I would expect that the KVR1333D3N9K2/4G (2x2gb) should also work, but I could be wrong. But the KVR1333D3N9K2 was also listed at tweakers.net's best buy guide in combination with this motherboard.

I think the problem is related to: ram, motherboard or gpu or a combination of these or a related bios setting.

Any advice about what I can try?

Is there an Asus program for your MB that monitors voltages and temps?
I have something called Asus PC Probe II that does that, but I haven't used it in a long time. Has alarms I think.
Might try that. Since it shows itself even when in BIOS I would first suspect power supply or the MB itself. Just guessing of course.