I can tell you that the Deluxe 90's we've seen have had their fair share of bad solder joints, bad jacks, and the usual common problem of the 5W resistors that desolder themselves from the PCB.
Your troubleshooting techniques are right on, you just haven't found the intermittent yet. Could be where a lead enters into a component. If the board is that sensitive in all areas, it's tough. And sometimes my friend, you just CAN'T find it. Those are the ones we all hate!