Mac Pro won't come out of sleep or power on - Heat related?

I bought my Early 2008 Mac Pro second hand towards the end of the summer last year and I noticed a couple of times that if I shut the computer down and tried to turn it back on again that it would click when I pressed the power button but not start. I found the only way to make it restart was to pull the power cable out and plug it back in again, then it'd restart. Sometimes I'd have to do this two or three times before it'd start working.

Over the winter, this problem vanished and I thought nothing of it, I don't actually switch the Mac Pro off very often, I normally sleep it.

A couple of weeks ago when it was very warm outside, it did it again but it stopped once the weather cooled down.

Last night, I put the system to sleep and a few minutes later I needed to get back on the computer so I tried to wake it up and it just clicked once and nothing happened. The only way I could get it started was to remove the power cable, wait a short while and plug it back in again.

So it looks to me that whenever the room temperature hits around 25C that my Mac Pro gets grumpy and won't restart. I use iStat Menus and the ambient system temperature still never really gets above 24-25C.

I've got an Apple 8800 graphics card, the default supplied graphics card (can't remember what that is!), 10Gb RAM and four hard drives in the system but to be fair, it did it last year when I only had the 8800, a single hard drive and 2Gb RAM installed.

It's as though a thermal cutout is getting the hump and refusing to start until I pull the power cable and reset everything.

Is this 'normal' behaviour or should I be getting it looked at under Applecare?

Thanks,

Keith.