I will also mention from direct experience the problems that a seemingly-good-but-really-bad HDD can cause.

I've had my self-built system for about 2 years now. Several months after I had it put together and broken in, I started noticed weird problems. Inconsistent lockups, weird software behavior, freezes needing hard reboots. This progressed and finally got to the point where I literally had to reset BIOS settings after every restart, because they system would detect things wrong AND it wouldn't save settings no matter what. Not only that, I was starting to fear that power supply was going bad and had caused me to already RMA the mobo once AND my expensive RAM. I was on my 3rd pair of RAM sticks and gnashing my teeth. All of this lead me to believe that my entire system was fried or about to be fried.

Finally, about 2 months ago, the HDD finally breathed it's last and started the repetitive clunk of death noise. Got my new 1TB HDD and plugged it in, and suddenly every problem I've ever had with the box disappeared. I mean every. Single. One. BIOS works perfectly. No random lock-ups after marathon gaming that heats my video card up to nukuler temps. No wierd RAM-based CRC or consistency errors. I've never seen anything like this happen in my 15+ years in working heavily with PCs.

All of that headache and heartache because of a single stupid HDD that was bad but kept working for a year or more.

So the short, short version is don't trust that HDD. It could be bad but still "working", AND it could be causing a lot of problems that may appear to be symptomatic of other hardware problems, like PSU issues. Just something to think about as you troubleshoot.