From my time in engineering, the first rule always was check the simple things first. You appear to have changed the mech fuel pump and not cured it, the fuel lines seem ok as you have changed tanks and not cured it. It may well be the fuel pressure sensor that is at fault, as Sherlock says, once you have eliminated etc,etc,etc.
You say there is no problem when the elec pump is on, does this not show there isn't any restrictions in the fuel supply lines?? Without looking at the drawings, how far away from the pump/s is the pressure sensor?
POST crossed with previous, didn't realise there is a possibility of air getting in to the pressurised fuel system, never heard of that problem before and I looked after two PA28140's for many years.
Last edited by PA28181; 5th Dec 2013 at 13:07.