excessive cranking on 1998 Mercedes-Benz CLK320

Have replaced all obvious parts but still have excessive cranking before starting. Have recently started turning on key, then off, then on again to start engine. This seems to be working right now. What needs to be replaced so I can just turn on the key and start the car

Sounds like a leaking pressure check valve in the fuel pump. This valve keeps up the pressure when you shut off the car, so you don't have to crank for a long time to build the correct system pressure. When you turn the key on and off you run the fuel pump a few seconds, but it will shut off when the engine is not running, so you can't just leave the key on to build pressure. I would have a shop verify this, though, because it means a new fuel pump which is expensive on your car!.

here are some shops:
I have replaced the fuel pump twice in the last week. Sent first one back as defective. What are the chances of getting TWO with the same defect? They are not Mercedes parts but after market parts. What do you suggest?