Check your fuel filters. Make sure that there is no blockage in them. In line filters, filters in the tanks. If that is not it check all your fuel lines make sure nothing is kinked or pinched. If that isn't it, I would think you have a pump going bad. I had a similar situation. Pump would work fine on AA battery but on the motor it wasent turning fast enough to pump the fuel in. I changed the pump and everyting was fine.

The pump I had a problem with was a Flight Works. I switched to a Haussel pump.

Joe