420 Engine revs up by itself then idles
 

It is a 1989 420 SEL with 144,000 on it. Beginning last winter, the engine will rev up from 700 to 2000 RPM by itself and then rev down back to 700 by itself also. I stopped using the car because it does it sometimes when I am stopped in traffic and the car would surge forward. Also, the reverse does not engage immediately. It takes sometime. I do not know if they are related. This car is very nice cosmetically. Thanks.