Ford F450 bogging down, running out of options!

1996 Ford F-Series Super Duty (Ford dealer says it's an F-450), 2WD, 7.5 Liter EFI, automatic overdrive. Speed density EFI (no Mass Air Flow). Runs fine sitting still, idles fine, revs up fine. Driving down the road it intermittently (but severely) bogs down/hesitates/backfires out the intake. It backfires out the intake multiple times when it acts up, and has blown off vacuum lines several times. Could go 100 feet or 20 miles before doing it, but it always does it. It will bog down for a few seconds, clear up for a bit, bog, clear up, bog, etc. Whenever it starts bogging, you have no choice but to pull over, it will not accelerate. Several shops have already checked into it and found nothing. We have been throwing parts at it for several weeks with no luck. Computer is OBD1 and shows no codes. We have had the trans checked out, and have replaced oxygen sensor (only 1), MAP sensor, both coolant temp sensors, fuel pump module, distributor cap, rotor, plugs, plug wires, distributor, ignition coil, ignition module, and the idle air control valve. No change what-so-ever, still doing the same thing. Timing & firing order are dead-on and the fuel pressure is correct. No vacuum leaks that I have found. This truck does not have (and never had) a catalytic converter, and the rest of the exhaust system is bone-stock and in great shape. I have checked the EGR system, it seems to be functioning correctly/normally. This truck does have an A.I.R. Emissions system, but I highly doubt that is causing the problem. Was going to try a PCM, but they are obsolete/not available. I know you can have them rebuilt, but I'm in a time crunch, and it would be a 6-8 week turn-around. I'm out of aces, I don't know what to do next! Any ideas?