I'd pull the intake line off the lift pump and blow some compressed air back through that line to the tank. That may dislodge whatever is restricting it. You could also temporarly plumb a line into a can of fuel to the lift pump and see if it will run like that.
You could have a bad lift pump... or a restricted line to the bottom tank, an air leak, a plugged vent line from the bottom tank or even have the lift and vent lines reversed...
I doubt it's a very serious problem.