I had a computer which failed intermittently. Memtest accuracy
depends on the version. In my case I had to get a late version
and run it for several days before it found a memory fault.
As to hardware faults inside a modern processor, that will be very difficult.
Maybe try running with some additional ad-hoc
cooling directed here and there. Make sure you have no power
supply voltage problems.
How old is the motherboard ? They can have issues with ageing capacitors.
Do you have any more information ?