Freezes and screen won't turn on after shutdown

Hi,

I'm having a lot of trouble with my iMac, and I can't figure out what's wrong with it. The problem is if I use the mac for a period of time, sometimes it can be almost a whole day other times it can be a hour. It all of the sudden freezes completely and the only way of getting it to work is to do a hard shutdown (holding the power button until it turns off).

But when I try to turn it on back on the the screen won't turn on. And I have to again do a hard shutdown, sometimes it takes around 5-6 times before the screen turns on. And if the problem occurred once and I try to use the Mac immediately after, it freezes within the first 10 minutes, and the whole procedure has to be repeated for another 10 minutes of usage. The only way to get it to not freeze up is if I don't use it for like a day, but that still only gives me around 3-4 hours before the problem occurs again.

I sent it to a repair shop to get it fixed, I figured that maybe its the hard disk. They ran some Apple test and told me it's not the hardware but the software causing the problem. So they factory restored it and gave it back to me. But it didn't fix the problem.

Any thoughts on what could be the cause of this?