if you have the crash dumps you can analyse them although that can be a bit of work and not always easy to tell what is causing it but for a few sugestions -
1. reseat ALL your graphics cards , pci cards and leads
2. blow all dust out of the case ( not really likely but always a good idea)
3. update your sound, graphics and directx drivers
4. update your motherboard drivers
5. rum a memory test program
Microsoft memory check
6. bit of a last resource but try flashing the firmware of your motherboard.