Computer only starts when hot, otherwise it is a pain to turn on, . . .

[Celery]

I think setishock has it right. It is an intermittent problem, a hidden problem. As suggested strip the system down to a bare case and check for misplaced standoff to start with. Start with the basic system meaning mobo, CPU and memory (and video card if needed) to start up with. Take it from there by adding a component at a time.

It's pretty rare to have a bad solder or a break in the traces but I guess that's possible. The mobo would have to be badly mounted for that to show up. That's why I suggested starting with a bare case so you can catch it.

 

[iPwn]

I'm going to have to go with Seti on this one. That definitely sounds like a broken circuit in the power button that is small enough to be affected by temperature.

Get a new case.

 

[setishock]

To test the power button unplug it from the header on the mother board. Take a small flat blade screw driver and bump the power header pins for 1 second. If it comes on it's in the power button or the leads. If that's the case check with the case manufacture about getting a replacement button. If they don't offer replacement parts then it's time to get a new case.
Antec P280

If it continues to exhibit the same symptoms, then proceed with checking connections.