First thing to check...heat.
I know, I know...your motherboard BIOS and your temp software say that everything is within range...that might be true...THEY WILL LIE.
As that heat dissipates off the CPU, VPU, RAM and HDD it must be evacuated out of the case fast enough so that it does not build up.
Eliminate the simple stuff first:
#1) check to make sure that all dust and crap are completely cleaned from ALL your fans
#2) Get all your cables nice and neat so they are not restricting air-flow.
#3) add another fan or two to your case that pull air out of it and if needed add one to pull cool air in.
You may want to check your thermal paste also....it may be too thick...or too thin or depending oh how long ago you did it it may have dried out...I check mine every 6-8 months just to be safe.
After you have done that. May want to reduce your overclock and start over rasing it slowly to see where the problem is.
Up your voltage.
I have the same motherboard (awesome!) as you with a 4000+ SanDiego that hit 3.0GHZ easy and an x1950XTX...heat monsters I tell ya!
I am on water, with 3 case fans 1 in 2 out all my temps seemed well with in range but I had random lockups and reboots...untill I added a 4th 80mm fan to the back that pulled out more hot air from inside the case....now it runs peachy keen even on the hottest days of summer without the A/C on.