That does not sound good.

I just have the usual stuff to offer: did you zap the PRAM? Is the PRAM battery good?

In the Service Manual for the 840 AV there is a procedure to reset the logic board (http://tim.id.au/laptops/apple/legacy/quadra_840av.pdf).

1. Unplug power cable

2. Remove PRAm battery

3. Press Power-On button of the Mac

4. Wait for 5-10 mins

Maybe that helps.