computer hangs problem

I am having a problem with my computer. It starts up just fine and will run for about 10 - 15 minutes without a problem then everything just hangs. If I am in a game the game may or may not continue to work for a couple more minuets but the sound goes weird. If I am on a web site I can do anything on that page without a problem till I try to leave the page and nothing will work. While this is going on the mouse continues to work just fine then even the mouse will hang and I have to hard shutdown the computer as nothing else will work. I have tried replacing the power supply thinking that might be the issue but no luck. I have also done spyware and antivirus scans as well as disk integrity checks on my drives with no luck. I built this system about 1.5 years ago and have never put the sides back on the tower. It does not matter how long the computer has been off before I try to use it, it will still lock up after 10-15 min even if it have been off for several hours. Any help would be much appreciated.
Thanks in advance
Dracos

 

Right click My computer, click manage, click event viewer, and check the little error triangles, often points to a problem.
I assume its not overheating, having the sides off, apparently is not always the help in cooling that is often thought. Have you done your spring clean (luff and dust)
Have you checked your ram - perhaps a faulty stick. Have you tried removing/replacing a stick at a time ?
I assume you can see the cpu fan spinning o.k.
Just the obvious things.
My favourite saying
Sometimes the obvious eludes us.

 

Heat was my first thought,also.
Sounds like the CPU is throttling down for "self-preservation".

 

Do you get the dreaded blue screen with any fatal exception or error messages or program not responding?