I do not think that the MB is the problem. I do had once the same problem with my Asus Sabertooth and FX 8150, the PC got frozen all of a sudden when under load.
I check the drivers, BIOS update, uninstall and install again the programs that happen most offen, over clock and reduce clock speed of the CPU and a few more. Nothing helped...
I did though reinstall my OS and it fixed, probably a lose registry value cause the problem. Try reinstalling the OS, this work for me I don not see why not for you since your problem is pretty much the same.
NOTE: You upgraded your hardware, same as I, before installing the OS perform a full format for the hard drive not during the installation for the DVD/CD but before.
Thermaltake Level 10GT-ToughPower 1500W-Frio Extreme/Asus Sabertooth 990FX/AMD FX 8150/Asus HD 6990/Kingston Hyper X Predator KHX18C9T2K4/16X /WD VelociRaptor 250GB(Main),x2 320GB(Raid BackUp)Caviar Black,2Tb Caviar Black x2/Creative SB XFi 3D Pro/LG Blu-Ray x2/Windows 7 64bit/Samsung T27A950 x3/Samsung HT-D6750W/Logitech G19,G9x,G13.