One thing that has not been mentioned: try a different scsi drive, or try the 840's drive in another machine. It may be that the drive's the problem.

I had this happen on a powermac once: the drive blew some small component on the bottom (as in, there was soot where it once had been), and the machine ceased to be - until I stuck a new drive in, that is.

The 840av does, however, have a known tendency to do what yours has done. Remedies vary: stick the logic board in the dishwasher, reseat all components, keep it running for 40 mins or so and then try re-booting. But if it has a bad scsi drive, it wouldn't be fixed by any of that, and replacing the drive would be an easy thing to try first of all.