All the above...and;
Your center vent vacuum diaphragm may be leaky or you may have a big leak in the system leaking away all your vacuum.
Best place to test is right behind the climate control.
I just refurbed my system. Resoldered joints in Climate control unit, changed center vent vacuum pod and replaced two open solenoid valves (1&3) on my 85. Check posts 92 and on. Start by removing the climate controller and check each valve electrically and with a vacuum pump. My 1&3 looked overheated. I suspect these are energized more often than the others.
Got a 1985 300D turbo
Your speed dependent hot air issue on the off ramp sounds like a monovalve issue. Take it out and inspect it for tears.
Good luck.