I had a very similar problem with my 94 E320 (only 80k+ miles), I had Merc Dealer do the 90k mile service and it cleared right up. I think it was the plugs, make sure they the right ones...resistor plugs can mess up ignition modules on older Merc's. The overall reistance can be doubled if you have the factory correct resistor built-in wires and resistor plugs..not sure it would run in that case but you get my point.

One thing I have learned in owning several Merc's, BMW's, and VW's is that you should always check the obvious first, can save you hundreds of dollars everytime. German cars are the best engineered cars in the world, especially the Mercedes in my opinion, and these cars are built accordingly. Properly maintained and driven, these cars can easliy amount 300k before anything really major happens mechanically.