This '89 Econoline 460... Is it an '89 Chassis Year, and is it EFI or Carbureted? Also, you mention "one tank" but is it equipped with One Gas Tank or Two? If TWO, the gas gauge runs through the Tank Select Valve and that could complicate diagnosis of the problem. My guess is Wiring or a worn-out spot on the "wiper" surface that makes up the connection to the gauge. Scary but true, the fuel gauge sender is a contact that moves according to a float, and rubs on an exposed surface in on the pump/sender bracket. All exposed to gasoline liquid and vapor.
Oh, I see you're in BC. First, congrats it's beautiful, but does this make your chassis an "export" or "Canadian" version of the Ford E350? I hadn't heard it does, but thought I'd ask.
