USB and PSU Problems RELATED

I am having a very odd problem, I have a feeling it is the MB but I am hoping someone can point me in the right direction to confirm this.

1) Starting having issues with USB devices not being recognized if they were plugged in during power up. Could resolve issue by simply unplugging and replugging once I was at the desktop

2) Starting hearing noises at different times from the fan near the psu. This has now stopped (YEAH- I THINK!)

3) Now sometimes the system does not even power up. I hit the front panel button over and over again. and nothing.

4) The really strange thing is that when I power down. The system shutsdown completely. Sometimes it will sit for 2 to 3 seconds, sometimes for an hour and it does nothing. Othertimes upon power down it will flicker on and off. Sometimes within a few seconds, sometimes hours later it will do this.

I starting look at the power switch for shorts. It is fine. I even isolated the switch from the MB where it plugs in and shut the system down, still flickers.

I then tested the psu-power supply unit with a psu tester. it shows accurate voltages. tested in for a few minutes and a few hours with no flickers or anything.

I am wondering if the motherboard is not bad, but i would like to somehow confirm my suspicions. can anyone point me in another direction with diagnosing this issue.

All diagnostics tools, running from windows desktop, including dell tools and other diagnostics tools show everything is fine.?!?

Please help