80 230E - New Fuel Pump still Screams?

[Dave Alex]

The car's original fuel pump began humming louder and louder as the years passed. The car is only started and run about once a month. This month the fuel pump failed completely. I removed and tested the old pump, dead. I replaced it with a new pump and a new fuel filter. I also drained the gas tank and inspected the fuel. Nothing of interest. On start-up the new fuel pump began to scream after just a few seconds of running. I suspected the aftermarket brand pump was faulty and took it back. A second pump of the same brand did exactly the same thing. I then paid the price for an original equipment Bosch pump. It too gets louder and louder the longer it runs and after 5 minutes the engine hesitates and will stall. Let the car sit for 5 minutes and the symptom will repeat itself. I am at a loss for what to look for next? Thanking you in advance.