I think siddharth... is on the right track. It sounds like an overheating issue.
Follow his suggestion and download hardware monitor (
HERE) and check your temps on both the cpu and the gpu. Also, be sure your rig is clean (no dust bunnies) and has good airflow. You don't say what case and/or fans you have, but as a rule of thumb I generally start with 2 120mm fans as intake (front or front and top), and 1 120mm exhaust (rear). Since you have the 212, go ahead and install it. Be careful removing the stock cooler as it may try to pull your cpu out at the same time. I've found the easiest way is to run a quick stress test to heat up the cpu/heatsink, then shut off the rig, remove plug from wall, press start button to discharge...