idle problem
 

I have a 1986.5 Porsche 928s, 5.0 V8, 32v, 4c, automatic trans. When I start the car the rmp fluctuates and will not hold a steady rpm (or idle) if the car is not warmed up for at least 5-10 minutes and the car is moved it will shut off but if more gas is given it will stay on but gives a knocking noise can't tell where it is coming from. After warming up the car will somewhat hold idle with a bit of fluctuation. I brought the car to a mechanic and he changed a sensor on the intake but didn't help. No mechanic seems to know what's wrong. Anyone have any suggestions.

thanks