A good scope diagnostic check by your mechanic should pinpoint, electrical related, i e cap rotor etc. Also if its fuel deleivery related.
Fuel distributor very($$$). I was going nuts with mine, rough smooth idle. Found two bad wires on 7 and 8 and others breaking down. Have it professionaly checked or try another mechanic. Parts changing and guessing way to expensive and many times its something minor.
1989 420SEL 105575 miles
1990 Chevrolet E-Cab C-1500
1989 Olds Toranado Trofeo'