Problems after Power Management changes

FIC P4M 915G/PD1 Intel P4 FSB533 HT ATI PCIE Display Adapter usual array of drives Musketeer Fan/Power Controller ? [hey... it came with the machine] Motherboard been running for about 2-3 years without problem. For convenience, I've always had the Windows Power Options set for "No Saving", i.e. leave everything on all the time. My conscience was **** by recent news reports of the combined carbon footprint of PC users who leave their machines running all the time. I went into Power Options and set the Display to go off after 2-hours and the system to "sleep" after 5-hours. Some days later I went to "wake-up" my machine but no joy! THE SYMPTOMS ARE: Hit the "Power On" button and all the fans fire-up, front-panel leds & lamps come on but as soon as I release the button it dies again. If I hold the power button on, power oscillates on, off, on, off continuously at about 1 - 2Hz. I HAVE TRIED: 1. clearing CMOS - no effect 2. removing all hardware a piece at a time - no effect 3. restored hardware but removed CPU - in this configuration the power supply starts normally from a single push of the power button? Likewise, switches off correctly after 4-second press/hold of power button. 4. replaced CPU with previously unused (i.e. out-of-the-box) Intel P4 and I'm back to the original symptoms... won't power-up from single button press and oscillates if button pressed & held. 5. Retained new CPU and removed Musketeer Power/Fan controller - no effect Finally, I don't get any beeps at all in any of the above mentioned configurations and I see no sign of bulging capacitors, pcb deformation or excessive heating anywhere on the board. QUESTION: At this point my guess best guess is a dodgy motherboard but if so, then how did this happen? Apart from overclocking [not possible with my system anyway] changing sotfware settings shouldn't lead to hardware failure! QUESTION: Could it be the PSU? Since it starts/stops OK without the CPU installed, is this really a possibility? QUESTION: is there anything else to try? Many thanks