Is it freezing without going to BSOD?
Are you using a page file? If not, turn it on for a while and see if you can get a crash dump...if you have that, solving the issue will be much easier.
Sounds like a cpu/memory/controller/board issue. This is very broad spectrum though, It will be difficult to figure out without testing many different things...If you can't produce a dump file; I would start with memtest86, then prime95 for cpu test, then onto furmark for GPU testing. Outside of memtest, though, it will be hard to isolate the issue as furmark/prime95 can replicate the same crash without giving you information on exactly what it was.
It sounds like a lot of work, but if you don't want to RMA the system again its where I would start; otherwise just send it back out.
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