Shutdown While Booting

Full tower computer will begin to boot and suddenly shut down. Happens with MSI mainboard and AMD 2600/ 266 front bus and DFI mainboard and 3200/400 front bus (TT Volcano heatsink/fan). This does not happen at the exact same point each time. I have tried a different motherboard, processor, power supply (400 and 600 W), power cord and memory (corsair). I have disconnected all devices except the hard drive and floppy. I can see nothing shorting out on the case. I am probably missing something simple. Can anyone help me?