Aurora R3 black screen crash

Hi all. My Aurora R3 has been having random crashes in Win 7 64 bit since day one. The crashes seem random and lately have become much more frequent. Crash results in black screen shut off followed by all fans running full speed. Memtest86 comes out fine with no errors. No HD errors that I can detect. all temps never get above 27 to 28 C. nothing is overclocked. When the crash happens I can not restart the PC immediately, even if I unplug, drain power, and reset CMOS. but...if i let it sit for a few hours it will restart. Need some advice on what to troubleshoot next...

Thanks for any help!