Navigator fuel pump troubles

Looking for some help on a continuing problem I'm having with my 2005 Navigator. For some reason, fuel pumps have been failing since I purchased it new in May 2005. Only use 93 Octane fuel and don't let tank get below 1/4 full! First fuel pump change in October 2006, then again in February 2007, and now the third one! No warning. Just sputtering on the interstate all of a sudden. Lincoln has been replacing them under warranty with no problem. Have 36,000 miles on the SUV. Dealership has no answer or suggestions, except maybe STP fuel treatment.

Do they replace the sock filter in the tank. Do you keep the fuel filter changed annually. Do the check the pressure to be certain the fuel pressure regulator is good. I would ask them to check the voltage going the pump. Once its out of warranty you need to trade the vehicle.