My son's 1990 240DL with LH2.4 recently started idling very poorly. The symptoms were very much like a previous incident that was caused by a bad pressure regulator. Basically the car will fire up normally and as soon as you put it into gear it will start bucking and nearly stalling. If you fiddle with the throttle you can get it going but it feels like it has no power until you get it up to speed (over 25mph or so) at which point it drives normally until you come to a stop and it does the buck and stumble routine and feels like its stuck in molasses when you start up again.

I brought it to the dealer convinced that the new FPR had failed but they said it was good. They suggested a new cat converter since they could hear some of the elements in the cat loose and also said I should replace the O2 sensor after which they could do further diagnostics.

My question is whether these symptoms sound like they could be caused by either a catalytic converter with loose elements or a bad O2 sensor.

Thanks in advance.