1992 190E Mercedes RPM up and down while in idle or stop

I have 1992 190E 2.3L Mercedes with 155K mile.I am scare to drive the car now while waiting for traffic light the RPM needle goes up and down anywhere from 500 to 1000 or little higher! I am not pushing gas but pressing the brake. It is not allways i have seen that. Few days back when i park the car in drive way then the RPM meter needle keep moving up and down looks like i am pressing the gas but i am not! Appreciate for your input and what kind of $$ amount i am looking at to fix it.

SPB1: There are quite a few systems that might cause this erratic idle condition. Are you sure your engine rpm's are actually going up? The Tach might be erratic due to some other problem. The idle speed is maintained by an "Idle speed valve solenoid". (Y6) Check the connector on this device, and its two large rubber hoses. It looks like a little cylindrical can with one electrical connector on it, plus the two big hoses. When the car is at 500-600 rpm's, disconnect the plug on the solenoid. If the idle remains the same, the engine control module or its RPM input is malfunctioning. If It still jumps from 500-1000 rpm, your problem is elsewhere. Connect a multi tester (digital) to the plug of the solenoid and see if the DC signal is waving back and forth too much. If it is going wild, I would suspect a bad OVP relay first, this relay powers all the important computers for the engine. Its a relay with a flip up top cover and has one or two automotive type fuses in it on top. These become defective quite often. (CHANGE THIS RELAY FIRST!) Your O2 sensor could cause this problem along with a bad EHA on the fuel distributor. You can disconnect the EHA also, and see if it helps your idle problem. If diconnectiong the EHA helps, its then a bad O2 sensor, or control module. (CIS-E or KE) Also check all your vacume hoses for leaks and repair them as needed. I would also pay attention to fuel pressure specs, and a good tune up to boot...
http://mb.braingears.com Go here to see where your system parts are located...

dave_rose69,

One more thing you brought up about the fuel pressure, I have seen while waiting in the traffic light or when the vehicle is park, the pressure also changes some time signficantly...which automatically changes the RPM as well. As you mentioned, I will start from OVP relay first...

Thanks a lot. Appreciate your input.

I am back again where i noticed oil pressure change while car is at stop or at park (engine running) and does RPM needle goes up and down!! I am not sure what is causing all this..As my previous question was also related with this looks like. I need your help and appreciated your time..