Booting up / recovering from sleep

Strange...

Recently my system won't wake up from sleep. Actually it won't start at all if I try to boot it.

It used to work fine, as soon as I moved the mouse, pressed a key, or the power button, it would wake up in a blink of an eye.

Now, if I try to wake it up, not much happens, I can see that the case coolers start spinning, but the flashing light indicating that the DVD is spinning, nor HDD activity seems to initiate.
Even if I then perform a shut down (press and hold the power button for 5 secs, unplugs the power or similar) and tries to boot it up again from scratch, the system just hangs... the fans will spin, but the drives doesn't startup. No post messages, no bios (no signal to the monitor.)

I have to wait at least 30+ minutes, but then it starts like nothing at all.
I have monitors on the temperatures, and nothing indicates that heat is an issue, it doesn't matter if I have been playing for hours, or if I just turned on the system. The CPU temp is under 50 celcius and the case is max 35 degrees.

I have been taking it apart, checking cabels several times, without it solving anything.

The power should be sufficient with a OCZ 700w GameXstream, its AMD 4200+ X2, ASUS AR832 MVP deluxe MB, 2 GB OCZ RAM, DVD, HDU and a soundblaster. The OS is Vista, even this shouldn't be relevant.

Does it make any sence to any of you??