What happens if you boot the computer, enter the BIOS, and just sit there for an extended period of time (hours to days, probably)? If you're having a CPU issue, you might expect to see the computer freeze eventually.
Other than that, when you start suspecting CPU, MB, or PSU, without diagnostic tools that most people don't have (because they're expensive and very specialized) you swap parts until the problem goes away. For what it's worth, your description makes me believe it's a motherboard or CPU problem, rather than power supply...
I'm not on the WHS team, I just post a lot. :)