Acer freezes; dying motherboard and/or PSU?

Problem
This PC (not mine) has been having some freezes. This has been going on for about half a year now and I just can't really find the source. As for BSOD's, there aren't really any. Most of the time it just randomly freezes. Since I only had freezes, I didn't really know exactly where to start looking. So I started to check the software, then hardware.

Troubleshooting I've done + Results
Clean Install
I first re-installed the operating system and did a clean install of Windows 7 (there was Vista installed at first (when the freezing first started to occur). Since Windows 7 has better memory usage and is more reliable. But without results. So I knew it wasn't the operating system causing the problems.
Memtest
I checked the RAM using memtest86+. which returned 120 errors. I then took out 1 of the ram's to test which one (or if both) were damaged. one of the ram had 100% pass while the other still returned the 120 errors. So I took out one of the ram's. But the freezing is still occurring.

Specs
It's an Acer M3641

Could this be the PSU dying ? (I read that OEM's PSU die quickly.) and/or the motherboard? and how can I check which one needs to be replaced? or if they are both damaged?

If you have a multimeter handy, you can test the PSU. Otherwise, the best course of action would be to swap out the PSU for another, see if the problem goes away, and if not, start looking into other hardware.

If you are unable to test the PSU, I would advise borrowing a PSU or buying one from a store or online store that will allow you to return the PSU for a full refund if it is not the culprit.

PSU Testing