RPM fluctuates and Engine stalls
 

Greetings All
I've a Merceedes E320 Model 1993
I'm facing the following issues with it:
1) I noticed that Randomly while driving my car when I slow down, the RPM starts fluctuating and eventually the engine stalls.
2) Also, sometimes while driving my engine stops suddenly!! After that I start the car, and the car starts normally.

Can someone tell me what is causing these problems? I would be really greateful for any feedback and replies.