The best suction pumps are the Gerotor gear types and not the vane pumps.

However seems to me if you had a suction problem it would occur from the very start and might also be sensitive to tank level.

As Denny said it sounds more like a voltage drop problem. Is the pump on a relay and/or not running through a toggle switch directly? Wires not too small, properly grounded with no looseness etc. The pump motor could also be sick; this would in fact tend to show up as it gets to operating temperature.

Kitz