Hopefully this will be my last post on this subject. After spending $500 on analysis that told me nothing, an air mass sensor, and an OVP, I think the final solution was to clean the contacts on my spark plug connectors (I finally found where Mercedes hid them - under that panel thing on top of the engine) and make sure there was good contact being made. You're probably wondering why I didn't try that first. Me too! The car is running very well right now (better than even before I really noticed the problem). I believe I'm going to go ahead and change out the plugs anyway just because I don't think they've been changed before. I'm replacing them with the same kind that are in there now (Bosch C8DC4's I think) unless someone tells me that that's not a good idea.
Thanks for all the input. Bob H