Sounds like the processor rather than the MB to me. I would be interested in how you wired the front panel, once you have it working. Try the manual reset, Hold down the little reset button on the MB for like 30-60 seconds and then try to boot it.
PB G4 15" 1.2ghz - Pismo G3 400 mhz - iBook G3 500 "Jukebox"
"SawSmurf" G4 500 - Quicksilver G4 Dual 1ghz modding...