Take off the heatsink and clean off the old thermal pad/paste and reapply some new thermal paste to the cpu die. Reconnect the heatsink and see if your system become stable.
Instructions for appling themal paste can be found HERE.
Check that your psu is putting out enough power for your system.
Disconnect anything not required to boot your computer. Just leave 1 stick of ram/psu/cpu/harddrive/graphics card.
Check that everything is correctly seated and connected.