RPMs too low when standing in Drive
 

I have a VW Passat 1.8 66kW, from 1992 I think.
Few days ago I started to have some problems. Car wouldn't want to start, you had to turn the ignition key and hold it a few times and give gas for it to get the engine going. After that in P(ark) the RPM would be nice - around 1000 or so, but when put to D(rive) the RPM would drop around 200-300 like the car would stop soon after. When driving ther seems to be no problems, but when braking to a halt the RPM go down again and only sometimes they would stay in around 1000.

What could cause this behavior?
I'm going to take it to a mechanics but would like to know first what "might" be wrong so that they wouldn't charge extra for something I don't need.

Could it be something to do with the air filter sensor, throttle body or sparkplugs?