The heatsink is 60 bucks from Small Dog. Your temps seem great, I cant say why its locking up.
Did you replace the heatsink compound? I used Arctic silver. The stock 120mm fan actually only
cools half of the sink. You can fit two thin 80mm behind the heatink. Also, the mdds, even the newer
ones are noisey as hell, you could replace the fans with Panafloes. What was the temp when
you locked up? You could bump the voltage slightly too.