that is probably why system was shutting down as not making proper contact between cpu & heatsink so cpu overheated.
try new heatsink to see if corrects problem. was old heatsink fan assembly properly inserted into motherboard on the 4 corners?
which means you had a big air gap between the heatsink & cpu. those clips, were they like hooks or barbed posts?
normally a boxed cpu comes with proper heatsink fan assembly so appears your cpu was not in sealed box
You did put some kind of thermal solution between the cpu and heatsink right? Whether it be a pad, thermal grease, etc? And if so how much did you use?