heh
setting the sleep and standby settings for the monitor and hard disks is probably the first thing i would suggest.
it sounds to me like this one machine that i had that did that when it tried to wake up from the first stage of sleep.
i disabled sleep, and it worked fine.
as far as heat being a problem, take your case open, and immediately after shutting it down, reach in and feel for a hot spot. try not to short anything out, so only touch the non conductive parts of the chips, and cheack the physical temp of the heat sink. if you do this as soon as it tries to restart, it will give you some insight on whether or not heat is the issue at hand.
also, make sure the heat sink is getting at least a little warm, i serviced a machine that someone had built at a shop, and i noticed it a little odd that the the temp sensor on the heat sink showed a nice and cool 28 celcius, and the processor sensor on the chip showed a startling 105 celcius. i figured something was lying, so i opened it and felt, and sure enough, the heat sink was cool. then i unbuckled the heatsink and fan, and the processor was hotter than hell. it turned out the heat sink wasnt correct for the socket, but after i ground off the corner that was in the way, it then came into full contact with the processor heat pad. after reassembling, it showed a nice 45 on the heat sink, and a nice 40 on the processor.
they then went and had the shop replace the processor, since even though it worked still, it had been through a higher heat range than it was warranted for, and they had caused it.
and the shop had simply turned off the overheat shutdown sequence when it gave them problems.
so make sure that it is right, and not just looks right.
~BoB~