That one is pretty old. I know that my idle was fixed once I replaced the O2 sensor in my 91 300E. I'm guessing that the idle may not be computer controlled so something may be sticking. In a lot of the later model cars, the idle air control valve gets gunked up and stuck causing idle issues. Also, try and find the water temp sensor (engine coolant sensor) on the engine and any air temp sensors (on the air intake) that could be old and sending wrong signals to cpu. Resistance increases with age which can cause a high idle. Other than that, old gunked up throttle linkages, springs, etc. Wish I could help but someone that owns your model will know a lot more I'm sure.