Thinking out loud
Others with the same problem have reported holes rubbed in the suction line inside the fuel module. should happen above 1/4 tank from what I have seen of the fuel module, but I suppose the check valve in the tank module could leak and allow the fuel to reach the same level as the tank. Basically the module is a bucket with a float actuated valve in the bottom. It lets fuel in but is not supposed to let fuel out. The VP returns fuel into this bucket, in theory keeping it pretty full. The module uses plastic/rubber lines that sometimes rub through on the modules slide mechanism. If the hole was above the level of fuel in the bucket air would enter the system.
If the check valve in the bottom of the module did leak air could get into the return line and possibly cause your trouble although I am not sure air in the return matters.
A fuel pressure gauge would help diagnos.