car slamming into gear!!

[85 500sel]

My car is slamming into gear all of a sudden. It used to jerk a little, but the other day I left it warming up and when I went back out leave the idle had not slowed. It was idling at around 1500 rpm's. When I put it in gear it slammed in reverse and drive. I drove it down the road and stopped. When I put it back in gear it did not jerk. Later in the day it done it again. Being that it always jerked a little I thought that it was transmission related. I posted a similiar post on Friday, but I only got one response. He said for me to check the idle control valve. Please help!! The car is a 1985 500sel. Thanks in advance.