car jerking when I take it out of park.
 

This morning I went out to warm the benz up and when I came back out to the car 10 minutes later I noticed that the car had not idled down yet. I waited another 10 minutes and the car was still idling high. I got inside the car and I noticed that it was idling at around 1800 rpm's. I put the car in gear and it jerked really hard. I put it in reverse and it jerked even harder. What could this be? I drove it down the road and put it in park and it went to 800 rpm normal idle. I thought it was just a fluke, but later in the day it done it again and again. Please advise me as to what is going on. The car is a 1985 500 sel. Thanks in advance.

Sticking idle control valve? I bet something is sticking....

where is the idle control valve? I really want to take care of this asap. I noticed that the car always had a little jerk when I put it into gear, but it never had a high idle like it did yesterday. Is the idle control valve something that gradually goes out?. Thank you pawoSD so very much for your help.