I have seen this behaviour in two 840av machines. In both cases, capacitor leakage was the problem. I washed the logic boards and both booted consistently afterwards.

Termination of scsi drives and scsi ID settings are crucial with any drive in a Quadra. The mysteries of scsi are not called scsi voodoo for nothing, however, so there may be some ritual, repetitive dancing and a fair deal of invocation of ye gods involved in checking and rechecking, setting and resetting these things. On an 840av, that, alas, is not an awful lot of fun.