Hi, 1980 volvo, B21F, 3speed auto. I have a problem that is really frustrating and can't figure where the problem is. I warm up the engine, idling great at around 900rpm in neutral, doesn't seem to matter how long I warm it up, but, when I put the car in gear, the idle drops to nearly zero, then pulses up and down between low rpm and around 900 rpm. Once I start to drive the car it goes away. Then, if I pull in someplace, let it sit for awhile, start the car, runs ok until I put it in gear, then the idle pulses up and down again, sometimes killing the engine. Happens more often then not.
I thought it might be the Aux Air Valve, tested it, looks like it's opening and closing ok. Even pulled the top hose of the Aux Air Valve, plugged it. Engine idles fine with it plugged until I put it in gear, then idle pulses up and down until placed back in neutral.
Any ideas of what's happening, what I can check?

 

Check all your vacuum hoses? Because whats happening is that your car is grabbing air from some place thats not metered and after the throttle body and your IAC is getting fooled. I'm assuming your car is kjet

 

Yes, KJet. I had the vacuum hoses replace a couple of years back, should be ok, though I will check again. What else can I check? Thanks