I had similar symptoms on my 1990 300CE:
the problems I fixed were teh following:
1.Vacuum leaks -replaced most of the vacuum lines and new rubber tees.
2. New Plugs, spark wires, distributor cap/rotor.
3. New (rebuit) fuel distributor as the plunger was leaking fuel. If you take off the air cleaner, and then depress on the sensor plate all the way, check and see if it is wet inside.
4. Replaced all the fuses and cleaned all teh contacts.
Car runs great.