Power Supply : Tiny LEDs On MB Light

when my PC is on, life is good. Everything works. No problems. Vista is set to shut down the power after 30 minutes and put the PC into sleep mode. At that point, the PC seems 'Off' until I hit a key/move the mouse; and it 'wakes up'. It seems like, sometimes, the PC will not wake up. when I hit the keyboard, moved the mouse - nothing. I hit the power button (held it in for 5+ seconds), hit the reset button...and still nothing. I tried unplugging it and plugging it back in. NOTHING. So, I opened my case and gave my PSU a fairly gentle tap and I noticed some tiny LEDs on the MB light up. At that point, I hit the power button - and the PC came to life. Vista said it was 'resuming windows' but everything was crawling about about 10% of the speed it should be. I had to reboot, after rebooting, everything was fine.

It does wake up sometimes though, sometimes it doesn't. When it doesn't, the only way I can seem to get it to turn back on is to 'bang' a bit on the PSU; but the power has never gone 'out' while I was playing on the computer (which is a lot of time). If the PSU was bad and there is a short or something and when I bang on it, it reconnects....I can't explain why it never 'shorts out' or comes loose while I'm using the PC.

If it were a bug with Vista, I could see how or why it might work sometimes and not other times, and why the power never dies while I'm using the PC....but I can't understand how my 'tap' on the PSU could get it to turn back on. Basically - I have no idea WTF is going on. A new PSU would be ~$100 dollars and I'd hate to replace it if there is nothing wrong. my tap on the PSU is not affecting the connection between the PSU and Motherboard. The PSU is at the bottom of my case and my tap is really not very hard at all.