Ok, first have a small house fan blow into your case without the side panel on, and see if it still happens...
Next, if the above doesn't help, try running it with one stick of ram at a time, and see if one of them makes it crash more often.
If that doesn't help, check the power supply...if you have a multimeter, you should test the volts, and see how much power you are getting(ask someone for more help on this, I have never tried it myself)