The thought that first came to mind with your latest post is that the engine runs fine on gravity feed - return line. Maybe it is a fuel pressure problem. During the rough drippy runs is this with the aux fuel pump on. If so, this might be the problem as the typical aux pumps come in a variety of output pressures. The one I have is Facet Part Number 40104 - 4 psi max 105 psi min - as shown on the Aircraft Spruce page.