I know that computer! Sounds familiar.
Ok, I would say that firstly check the thermal paste, but I KNOW it was applied correctly and it was a AC thermal Greece.
I would say that it is most likely the water cooler that is goosed, highly unlikely to be the CPU or board as generally they don't break.
If it was me I would buy a cheap universal air cooler. Strap it on and run heavy load, carry out a bios default reset to make sure all alerts are set correctly and that it shuts down correctly to prevent any damage.
Hope this helps