Hey John, sounds like your fuel sending unit may be periodically sticking in the tank. In my research, people have said that a good fuel cleaner may take care of the problem, otherwise you'll have to replace the unit.
Unfortunately, my situation just got a little worse. As I said in my original post, the seller lied about many things - he also failed to mention that he replaced the dash board to show lower mileage and may have disconnected the check engine light. When I tried to get it inspected, I have four sensors not available. In an attempt to obtain a waiver, the history of inspection showed that my Expedition actually had 337K miles at last inspection, rather than the 159K that was showing on the odometer when I purchased it.