1988 Chevy Truck rough throttle acceleration. Now IAC goes

Engine performance problem originally started out as horrific stumble with occasional backfire when giving throttle AFTER engine is at normal operating temp. When engine is cold will run smooth at idle AND when given throttle. Once engine temp. Reaches approx 180, engine will run smooth at idle however will stumble, cough, buck, backfire when giving throttle, especially when under load (driving). Smelly exhaust. No codes initially. However, eventually did get a code 45, (rich fuel mixture). Code has not reappeared since cleared. Today, same problems, however, after letting engine run for about 20 minutes and periodically goosing the throttle, the stumbling has stopped when throttle is given. Now however, when you put truck in gear the IAC immediately increases from 35 to 145 on SCAN tool. Engine temp. Goes from 190 to 215; and because IAC has gone so high engine revving at 2500 rpm with no application of throttle. Putting trans in neutral IAC drops back down to about 45 and eventually coolant temp will drop back down to 190. Originally thought throttle position sensor (TPS)was a problem. Lowest it reads is.62 v, but does climb when throttle is given (max reading I have seen on tool was 2.5v), but does seem a bit sluggish. Maybe supposed to be, I don't know if there is an intentional time delay built into the output of this device. I bought the TPS to replace it, but once the IAC started its immediate "jump to the moon" when trans is taken out of park, I'm surmising it really isn't the problem now. I've read other inquiries made regarding oxygen sensors and MAF. Tool reads O2 measurements which regularly switch from "lean" to "rich" to "lean", etc. Which I understand it is supposed to do. Guessing MAF at this point, but since the problem "changed" I'm at a loss on this one. What do you guys think? MAF, O2 sensors, TPS? I have no doubt computer and IAC are doing their jobs. Any guidance appreciated. Truck computer/ throttle body performance has been exceptional up to this point. But is not "drivable" as it is now.