What PSU do you have ?
If you totaly power down the system (remove the power cord / hit the switch on the PSU), does it make a difference when booting up again ?
I had simlar issues and it turned out to be a dodgy (q-tec
) PSU being arsey with the standby voltage. A Tagan later and it was sorted.
The issue wasnt exactly the same, but close - it occasionaly wouldnt boot with a USB mouse + keyboard, but would always fail to boot with a USB modem plugged in (assume it was drawing too much).