This problem was widely discussed back in 2007 & 2008 & seemed to happen on other boards too. But I did not find an explanation for the problem or a fix? Surprisingly, a suggestion to momentarily turn off the power supply temporarily "solved" the problem after struggling with it for 2 months as described below.
Any thoughts or suggestions would be appreciated.
My Asus P5K-E/WiFi-AP mobo has worked perfectly for 5 years & still does - when it successfully POSTS. I have been running 32bit Windows 7 w an E-6750 CPU, and do NOT overclock.
The problem first appeared two months ago when the system intermittently fail to POST (no beeps). At first it would usually POST on the 2nd or 3rd restart (after holding down the POWER button for 4 seconds). But after a couple of weeks, it wouldn't boot unless I warmed the motherboard with a heat gun, or left the system on (in the "NO POST" state) for 10-15 minutes before restarting it.
During that time I swapped the Power Supply & Display Adapter, but the problem remained. I also ran extended memory tests in Windows 7 & found no problems.
Then for 1-2 months the computer worked perfectly and always booted on the 1st try from a SLEEP mode. It would also reboot on the 1st try immediately after a SHUTDOWN unless the system was left OFF for a period of time. (long enough to cool off?)
But last night the system crashed (turned itself off), and could not be restarted even after letting it warm up in the "No POST" mode. (I did not try the "Heat Gun" method.)
But the suggestion to momentarily turn OFF the Power Supply "fixed" the problem & system booted on the 1st try!
I have this same board and it has been great for 5 years too. I do use a custom bios linked below, which has just been updated again. To me, it sounds like your board is having issues with thermal expansion. I never turn mine off. I have a hard drive in a router that is over 10 years old too, I think it helps the life.