I ran a dyno during that cars reign. They did suffer from rough idle for several reasons. The valves were particularly prone to carbon at the valve seats, usage of good fuels like Chevron with Techron seems to have solved that issue. The fuel system is a CIS system with fuel distributor. The injectors had a tendancy to become fouled with age causing undesirable spray patterns. The spark plug wires had a weakness in the resistor ends and did break down causing secondary leakage. I would probably run two fuel tanks with a bottle of Chevron Techron in each tank and then re evaluate the running. If it is still poor you should have it checked with a scope to evaluate the ignition wires and basic function. The injectors are mounted in plastic cups with seals that also cause vacum leaks and poor idle quality. Just some ideas, some of these items are quite expensive so the fuel treatment is where I would start.
Bill