1992 190E 2.6 mixture problem
 

I have the Lambda screw set so that the duty cycle on a warm engine is 49% at idle, initially. After a few seconds of idling, the udty cycle begins to drift down, to about 20% or even less. Turning the engine off and restarting seems to "reset" it back to 49%, but then the drift begins again and I can tell from the smell the car is getting really rich. On the road, there is an initial stumble, but once above 1500 rpm the car drives acceptably. Does anyone have a suggestion as to what to troubleshoot? To add to the problem, I need a new bulb for the check engine light, so I don't know if it would be coming on or not. Obviously I need to correct that.