Electric fuel pump

I have a 49 mercury,flathead, 6-volt,positive ground with and electric fuel pump. The pump pushes fuel in both accessory and ignition positions, however when the car is running, the pump stops working and fuel pressure drops to zero. I have used this pump for years and have replaced it with a new one, but the problem persists. It is hooked up with the hot wire to ground as the manufacturer suggests. Any thoughts on why this is happening?
Help!!! Bobby

Could it be a problem in the fuel line between the pump and tank or the tank siphon tube. If there is a pinhole or larger leak, the pump could be sucking air which will stall the pump. Or, are you sure the pump is cutting off?

Sorry, but life got in the way of this issue, but Now I’ll explain. The pump(6-volt) on my 49 merc (pos ground) is well grounded to the frame. The neg. wire from the pump runs to the coil post on the switch (start&run-this has a start button). The pump works fine in the start position, but shuts down when the engine starts. When the pump is connected to the acc. post on the switch, the pump runs on the acc. position and the start/run position, all dash gauges work, but the pump still quits when the engine runs. I have to assume that the switch and pump are not the problem,but that it is somewhere before the switch.Any thoughts would be sincerely appreciated.

P.S. when I hooked the neg. wire from the pump to the battery, the pump works fine while the car runs.Go figure!