Interesting problem. First of all, you might not know that the front switch on most newer computers only sends a signal to the MB to turn the computer on. If you get forced into a hard lockup situation it will often be impossible to turn off the power with this switch. If you hold the switch in for 5 seconds or so, that will turn off the power.
Now, to the problem itself. I would first check to be sure that your video card, CPU, and all periperal cards are firmly in their slots. I've fixed a couple computers with similar symptoms where the problem ended up being that the CPU was pulling out of its slot when the system warmed up.
Next, you might check that your sleep button's connector hasn't came loose. (I've never seen this, but I suppose it could happen.)
If nothing above works, I would probably try a power supply first. They are cheaper than MBs and you can probably return them if they fail to fix the problem. (IMO though, I think it's probably the MB.)
Good Luck
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bloom25
THUNDERBIRDS
