First thing i did with my truck when i got it was update the entire fuel system. Removed all the factory line, removed both tanks, removed factory fuel filter and lift pump. Installed a 38 gallon bronco tank with a sump, ran all new CPE nylon braded fuel line, installed a E fuel system, and thats what ive been running the past 6 months.
The factory fuel system is prone to air intrusion. Start at the tank and work your way forward inspecting everything. You could have a leak at the sending unit head, a clog in the pickup tube, or as mentioned about, the selector valve is a huge issue.