car jerking when I take it out of park.

[85 500sel]

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.