Yeah checking temps right after a restart will let them cool down drastically in the short time to reboot. The best way to do it would be to run a stress-test on your GPU/CPU with something like CoreTemp and GPU-Z running simultaneously so you can monitor how high they get before crash
If temps are still fine and no crash, maybe check your RAM with Memtest 86+
I would Follow NoUseForAName advice this is basically the best way to check
For your CPU stress test you can run Intel Burn Test v2.53
http://www.majorgeeks.com/files/details/intelburntest.htmlfor GPU you can run FurMark.
http://www.ozone3d.net/benchmarks/fur/Also when your run the GPU Burn In test make sure you have a Custom Fan Profile with MSI Afterburner or eVGA Precision..
And EVERY PC or Console Colloects dust, depending on environtmet some are mostly not much dust but if you smoke, have pets it can get rather filthy in there.. Unless you live in a Dust free Environment you are collecting dust..