I have a PC that sometimes won't boot, this happens when it's been off for a few hours, the power supply turns on and the case fan, cpu fan and power switch lights turn on, yet there's no beep, no video. The power switch and reset switch don't work, keeping the power switch pressed doesn't turn off the power supply. The voltages are OK, even power good is good although a bit lower 4,88v instead of 4.92 when the PC starts well. This is a new motherboard, I've changed the power supply for a good one but the problem remains. Eventually the PC will obey and start, as if it needed to be a bit warm to work well.
You may have a bad power switch on your case, or a loose wire behind your plastic bezel where the case leads are. I would check those next. I removed my bezel recently and broke off my reset wire, so it's a possibility.
Hey, rodion15, I've experienced a similar-sounding problem recently. My advice to you is -- painful though it may be -- to rebuild your PC from scratch, ideally out of the case at first to eliminate that as a potential source of a short circuit. My machine was powering up sporadically until I took everything out of the case and started again.
Well..I've tested the bare motherboard as johnh83 said, RAM and CPU only, no keyboard nor mouse, and the problem remains: I try to turn on after a few hours turned off and the problem is there again. It seems clearly the mobo, unless it's caused by the RAM stick or the PentiumD, but I don't have a spare cpu or ram to test. I'm going to return this mobo.
o1die: I suspected that too, that's why I removed the motherboard and tested it bare, thanks for your answers .