Computer not starting, except sometimes...?

[TacoRaptor]

So I wasn't sure exactly where to put this, but my PC is having a horrible time with starting up lately. When I press the power button, the entire computer "blinks" on, and then shuts right back off. Like within a second. If I keep trying, the same continues. I have to either unplug and then re-plug in the power cable, or switch off the PSU then switch it back on again until it eventually turns on. Today there was a new development though.. the GPU showed no signs of power when the system was off. Usually there's two LEDs where the connectors are plugged in, indicating they have power, even when the system is off. Those were dark when I went to turn the PC on today. So of course when I hit the button, absolutely nothing happened this time around. I flipped the PSU switch, and the GPU lights flickered, but turned back off. Tried again, and the same result happened. Tried a third time, and they stayed on, but the PC did its "flicker" thing, so I had to try again, and this time it started. I've talked with some friends and we think it could maybe be a motherboard or PSU issue? But I'm at a loss. I've been told it could also be numerous other things, so I thought I'd post here. Any suggestions would be appreciated! Also, my specs are on my profile, if needed. Thanks!

[TheGav]

Sounds like the psu