TL;DR: My computer freezes randomly, but if I run Windows memory diagnostic, the computer works fine when it boots, even though the diagnostic tool reports no errors found...is it my RAM or is it the motherboard...or something else?
More in depth:
Some time ago I changed the motherboard, CPU, PSU and GPU on my computer. After I did this, my computer started freezing at arbitrary points; when just sit browsing, when I play games, even when I just listen to music. Sometimes sound stop, sometimes it does not. Sometimes only certain programs stop responding, sometimes everything freezes. I've also experienced irregular BSODs with various error-codes.
Eventually I tried re-installing Windows (8.1), and everything was fine for a few weeks. Then it started freezing again, and a few BSOD have occurred. A few days ago I got tired of this, so I decided to look more into it. I started by running Windows memory diagnostic, which reported that no errors were found. I ran 1 extended test and 10 standard tests.
After the computer booted, it didn't freeze again. The next day when I started it again, it started freezing. So I started GPUZ to monitor the GPU. The log didn't show anything strange with the readings from the GPU, but when the computer froze, the logging froze too (30 second holes in the logging). I decided to try to run a memory diagnostic again, and when it booted it didn't freeze again. Today I ran the memory diagnostic again, and everything works like a charm.
So, is my RAM messed up? Should I just buy new sticks? Can it be the motherboard?