Games don't stress-test components very well, it could take hours or days before anything happens. Even if RAM is damaged. Had that problem so I know. Could take 2 days for my comp to crash but memtest86 was spitting out errors after 10 secs.
Same with overclocking, games dont stress the system that much.
Try OCCT for a few minutes at least. 3 runs of 5 minutes each should be sufficient.