First thing I'd do is change the O2 Sensor if you haven't already. This made the biggest noticeable diff on my 91 300E. You can use the Bosch one made for the 95 Ford Mustangs and just splice the wires in (it's same O2 Sensor but without harness and available at Kragen, Autozone, etc.). Next would be to replace the coolant temparature sensor at the back of the engine near the firewall. Then, replace the Air Intake temp sensor on the air intake tube. I also replaced my rotor, ignition coil, plugs, and OVP but I don't think I notoced anything when I did that. I'm thinking it has something to do with a sensor because when it gets hot, it starts running worse which may be the O2 sensor sending bad readings from the exhaust. After that it could be anything, especially of someone messed with the Lambda (air mixture) adjsutment.