I had posted a couple of things about this on the "spark plugs" thread or another one, not sure, but the issues there dont seem to match mine exactly.

The car is a 1988 Mercedes 300SEL with 158K miles. Dont leak or burn oil.

My problem is that I have this "spluttery" idle when in drive and the idle would occasionally go up and down... even when in gear as if stopped at a stop light and I have to depress the brake harder to keep it stopped. Recently I have noticed on acceleration that the cars power to the transmission is "uneven", kindof like the car is getting more power, then down a little, as if I am "working" the pedal, when I am actually holding it steady. Sometimes when in park the idle will go up and down some. The "check engine" light will come on about 30 seconds after the car is started and will go out about 90 seconds after that and never comes on again until turned off again. Someone said maybe o2 sensor is bad... would that cause this? Can a place like Advance Auto parts store check this cars CEL code?

I dont have a ton of money to check this and that and wondering if anyone here can recommend any specific thing that may be causing this? Perhaps the spark plugs need changing? Other than that, all I know is to keep driving it until it gets worse and dont run anymore.

Thanks in advance.