dell dimension 5100c diagnostic lights flickering

Unusual issue: have googled and searched these forums with no success.

Dell Dimension 5100C will no longer power up.  When power button pressed, power light is solid green as is green led on mobo.  However diagnostic lights 1, 3 & 4 and the Hard drive light simultaneously fast flicker until unit shuts down (4-5 seconds), then automatically tries to restart with same result except the shutdown occurs in shorter durations.

No external video card present.  Have tried reseating memory chips; installing different memory chips; running with no memory; disconnecting internal hard drive; cd/dvd, floppy drives; removing the CMOS battery and the connector to the front panel i/o board to mobo; all of these at once; resetting cmos by moving CMOS RTCRST jumpers from 1-2 to 2-3, then powering on, and returning them back to 1-2; inserting external video card; all with virtually the same result.

Ideas?

Thanks