I hate to sound like those people who say, "Maybe if I just let it sit for awhile it will fix itself," but...

... I have revived a couple of mysteriously dead Macs (no power at all, or chimes of death) by unplugging everything from the motherboard - PRAM battery, hard drive, floppy drive, RAM, VRAM, CPU, anything not nailed down or soldered on - and letting it sit for several days, and then reassembling it.

Sounds weird I know, but perhaps worth a shot before you dumpster the mobo.

M