S-Chassis.com - 93 240sx dies on deceleration

93 240sx, base model, 5-speed, 197k miles, cylinder compression within FSM specs.
I have a Factory Service Manual but no ready access to a Consult tool
I've checked the ecu and currently have no hard faults

The idle is normally stable at 750-800 rpm when warm, w/o A/C running.
When driving the car, the idle will fall below 500rpm when decelerating and will frequently stall. If it doesn't stall, the IACV/AAC will eventually regulate the idle back to 750-800. This problem only happens when driving the vehicle. I can't replicate the problem in the driveway. I've tried revving it and I've tried running it at 2-3000rpm then releasing but it decelerates and idles just fine. I've also tried the same experiment while holding the brakes to see if the brake booster may be involved (no change).

The IACV-air regulator (bypass) works properly to increase cold idle, then closes when warm.

The IACV-FICD solenoid increases idle to about 1200 when I turn on the A/C but the IACV-AAC will cut back the idle to about 850-900 after a few seconds.

The power steering pressure switch functions correctly, increasing the idle when turning the steering wheel at idle.

The FSM describes a Vacuum Control Valve in the IAA Idle Adjusting Unit. The description is vague but it sounds like a mechanical vacuum relief valve. I could use a vacuum guage to test it but I can't find any documentation on its release setting. I suspect that if this vacuum relief were stuck closed it could be causing the problem but I'm looking for guidance.