Mercedes Benz repair: 190E Engine Missing & Stalling, fuel pressure regulator, fuel pump relay

Question
1990 Mercedes 190E 2.6. About 270000 miles.
After sitting all night, it starts without a problem, and runs fine for several miles or about 15-20 minutes. Then it starts to miss, buck, hiccup, fart and generally try to die. If it does stall, it takes a lot of cranking to get restarted. If I am on the highway, or if I can keep the RPM's at or above 1500 for 10 minutes or so, the problem resolves itself and the car runs fine for the rest of the day. This happens pretty consistently.

Answer
These are some of your possible issues:

Alternator
Voltage Regulator
Fuel Pump Relay
Overload Relay
Ignition Coil
Alarm Control Unit
Reference Sensor
Fuel Accumulator
Fuel Pump
Fuel Pump Check Valve
Fuel Tank Vent Valve
Altitude Sensor
Cold Start Valve
Cold Start Valve O-Ring
E.H.A. Valve
E.H.A. Valve Seal
Fuel Pressure Regulator
Idle Control Valve