The chassis itself can possibly be a significant contributor to problems; in future, you should remove the board from the chassis for isolation tests. It's a pain I realize, but...
Have you gone through the troubleshooting processes suggested here: http://www.intel.com/support/motherboards/desktop/sb/cs-010254.htm? Specifically, have you tried resetting the BIOS configuration?
The SO2 board has a 2-character hexadecimal post code display on it. What are you seeing happen on this display when you power on?
Hhmmm, I am racking my brain here. I know I have seen this issue before but it's been so long that I don't remember the possible cause(s) .
You said that you tested the PSU. How? Did you try substituting another PSU and see if the operation stayed the same? If not, try this - and take the board out of the chassis when you do so. If this was being caused by a ground loop, for example - which could occur if the PSU is starting to fail but hasn't completely - taking the board out of the chassis and testing on a grounded surface could yield a difference...