All of a suden CPU over heat error on boot

For the CPU to overheat that fast, there must be a problem with the way the heat sink is installed, or the CPU's fan isn't working.

Start by checking if the fan is tuing. If not, check its coection.

If it does tu, it could be that the heat sink is not seated properly or you haven't installed new thermal paste properly.

You should consequently take it off again, reapply thermal paste as directed in the package, reseat the heat sink tightly, and try again.