Exactly how aggressively did you overclock your rig? CPU, GPU, (and RAM if you changed that too).
If your GPU has bad bearings on the fans, have you at least tried removing it and running the system on on-board? Would help rule that in or out very easily.
-If you're certain your PSU is fine than you at least owe it to yourself to put it on a PSU tester, or try an alternate, known-good PSU to know for sure. "Feeling" that you psu is fine isn't enough when you ask the internet for help.
If the system does the same thing with the GPU removed and with a known-good PSU (be best if able to try a diff psu or at minimum a rigorous PSU test on yours, including +12v rails, the CPU 4 pin and GPU connectors) than I find it most likely you'd be looking at a bad motherboard at that point. Do what you will if it comes to that (not sure eBay price of that board vs just being time to get a new cpu, mobo, and ram).