Check every rubber jumper hose on the suction side all the way back to the tank, especially the one at the sender. It is higher than the tank so will not leak, but the pump will suck air there if the hose is old or if someone installed gearclamps there; one on each side, instead of the oem bandclamps.
If you have to use gearclamps, install the correct really small ones and use two per side with screws rotated 180* from eachother.
