with EST disabled, timing jumps back and forth from 10deg to 20deg, what would cause

 

How does the idle sound? Are the RPMs jumping around or is it nice and smooth? Is the rest of the ignition system fairly new or all old and probably needs to be replaced? It could be a number of things ranging from needing new plugs/wires, etc to a new distributor, vacuum leak, bad timing light. We need some detail on the current condition of the car and the parts on the car.

 

I'll second Dave on the timing light .

This is because I watched a friend go bonkers chasing a fluctuating timing situation , the poor guy changed everything but the kitchen sink , and , you guessed it , it was the timing light itself that was defective !

If your car shows no other symptoms such as a wandering idle , try a different timing light just to verify whether or not the timing is actually jumping .