Volvo Repair: 1982 volvo 244GL erratic idle backfire, engine wiring harness, speed motor

Question
On my way home cruising idle started to bounce from 2100 to 2400 and down to 1800 or close to these numbers, when at a light my idle was minimum 1400 and jumping up to 2400+ and back down and all over in betwen plugs wires cap and rotor are all newer items, this came on all of a sudden. what could it be?

Answer
Joe, I've seen this before and it turns out to be inside the engine wiring harness.  Over the years the wires get chaffed and bare wires touch each other.  Harness for that car should be about 350 bucks.  What you may want to check also is the hoses at the Air speed motor.  I would also tap the motor with a hammer and see if it changes the idle speed.  Roger