My 1975 450SEL idles all over the place. Generally after cold start it idles @ 1100 RPM. However when I restart after about 1/2 hr, it idles @ 500 RPM or less and often stalls. Very unpredictable. What can I do to at least increase the speed so it does not stall? Any and all help appreciated.

75 450SEL
89 190D