Likely heat related as the others says. If it's not the graphics card, you have to look elsewhere. PSU (Power supply) fan, motherboard fan, cabinet fan etc. are things to check (the ones you got out of the list).
Anyway, steps to take:
- Download diagnostics (I suggest the Dell one even if you're on a non-Dell system): http://ftp.us.dell.c...gs/cdd_1291.iso
(then burn it to a CD with a CD writing program that supports ISO, boot from the CD and start the diagnostics)
- Download Ubuntu Linux and boot it from the CD. Test some demanding applications. If they do not crash the system, it's likely software related): http://www.ubuntu.co...ubuntu/download
- Check for fan statistics in BIOS (or with a program like the kitty suggests
- Reset CMOS (CMOS being the memory storage for BIOS, witch is the basic hardware settings etc. for your computer). Clearing CMOS will set your system back to what it was when you bought it (so if processor speed or something has been set too high for some reason, it's set back to "safe settings"). This may cause slower performance however (setting the system back to "safe settings"), so do it at your own risk
Those are some of the diagnostics / software steps. For hardware:
- Check all fans as suggested (that they spin fast and that there are no dust or other obstacles in their blades)
- Check the capacitors in the PSU and motherboard (faulty capacitors will cause all sorts of errors, freeze / hang being one of the more common): http://home.earthlin...eli/index27.htm
- Reseat all components (video card, cables, other cards etc.). You'd be surprised how often that can solve problems (having worked for support myself, I know!)
- Test your system with various components disconnected (DVD, video card etc.)
If none of this gets you closer to solving the problem, let me know.