Memtest86 6.0v - random errors. What part of pc failing?

[asia23fab]

Hello. I have a strange problem. Sometimes memtest86 pass 12 hours without single error. Sometimes it throws errors after 5 minutes.It seems totally random. I dont know what part of pc to change now. What can it be? Memory or other component?
It can be broken memory controller in cpu cause errors in memtest are very random ?
Also games is crashing sometimes too.
SuperPri , Prime95 goes without errors.
I am in big hole now and i am very mad.
Also i am using memtest86 6.0v

pc:
cpu - 4790 stock
ram - corsair 8gb 1333mhz 1.35v low v.
mobo - Asus Sabertooth Z97 Mark 2
psu - Corsair 750 Rm

[David (PassMark)]

Maybe there is a real RAM error but the frequency is low. e.g. one error every 10 hours on average. Which of course makes it harder to confirm any possible fix.

Without seeing the actual errors, the frequency of errors and the logs, I would initially assume it is the RAM at fault. Do you have some spare RAM to test with?