What tools to test the reliability and stability of a newly built PC?

prime95 ,it's widely used for stress testing both the CPU and the RAM

These software solutions are too unnatural. This is how they test in labs, and it would not surprise if one will ever find hickups with these tests. With any Intel processor, you cannot kill the processor, even if you have bad ventilation. I would say: use it!. And have your 4 year old kid use it. That is a much better stress test.