I'd load up one of the temperature monitoring apps and verify that the CPU overheating is actually the issue.
Heat sinks will get hot even in normal operation.
Did you put thermal paste on before remounting the heat sink?
The reset could be any number of things.
no i didn't had any thermal paste so i just cleaned it and mounted again. The processor was not sitting with the heat sink properly so used some sand paper to make both of them flat and wow it ran successfully for 2 hours.