1988 Volvo 760

After engine reaches normal operating temperature, around 180 the engine rpm is extremely erratic. Engine rpm speeds up and down in neutral and under a load making it difficult to drive. A little history. After the volvo was driven and the engine was shut down after being hot, it would not restart. No spark. It would only start after the engine cooled down. Replacing hall sensor( distributor ), distributor cap, rotor, and spark plug wires. Seemed to fix the hot start problem. The following day I drove the car to work and I started having this erratic rpm problem after the engine got hot. The vehicle starts whether it is hot or cold, but rpm becomes erratic after engine gets hot. I have repaired several vacuum leaks, and tested engine cooling temp sensor. Resistance seems to be within range both hot and cold. I don't have the speciality tools nor the expertise required to troubleshoot other system related components. HELP!