PC intermittently does not boot

[timread]

PC is as in my signature. It has been running fine for 2 years. However recently it has started not powering up the first time I turn it on from cold. To fix the problem I have to hold down the power button for about 30 secs, then briefly press it again - at which point it usually boots up.

When the PC is stuck in its 'non booting' state, the Power and Reset buttons on the Asus motherboard are lit, so it definitely has power. I have also tried booting the machine using the motherboard Power button, and it does not boot (i.e. no difference from using the power button on my case)

I think I have narrowed it down to an issue with the motherboard, but before I go out and spend money replacing it with another semi-obsolete Z68 or Z77 board, does anyone here have any other suggestions?

Here's what I have checked already:

Reseated all components on the motherboard
Checked for short circuits between motherboard and case (none I can see, all stand-offs used correctly)
Tried a different GPU
Cleared the BIOS to default settings
Took CPU back to stock speed (had been running a mild overclock using multiplier only, never over-volted)
Voltages on my Seasonic PSU, both in the BIOS and monitoring in Windows, looks correct and stable on all voltage lines
Temperatures of CPU and other components are fine, nothing is overheating

I'm going to try a run of Prime95 and Furmark tonight to see if putting the CPU and GPU under load introduces any instability/BSOD - in order to rule out this being a CPU problem.

I also have another PSU I will test (although my gut feeling is that this is not a PSU problem)

I'll update this thread later with the results.

In the meantime - does anyone have any suggestions?

[shaithis]

Change the CMOS battery, flash an older BIOS and then flash the newest back over the top.

Failing that it may be a slightly cracked trace, in which case I hope you know someone with a reflow bench.....those Z68/Z77 boards are getting hard to find (or damn pricey!)

[timread]

Good suggestions, thank you. I forgot to mention, I have replaced the CMOS battery (the old one still tested fine though). I'll try the BIOS reflash.