I had a fuel pump assembly replaced by my local shop on my 06 Ram 1500. Filled the tank after the install the gauge stopped at 3/4 and then about 5 minutes later it slowly starts to move to full. Took it back to the shop and they put a different Mfg. pump in. Still the same thing. Took it back a third time and they manually worked the sending unit while watching the gauge and they tell me it works properly. They put it back in the tank and still doing the same thing. Any idea what could be causing this?