Whatever it is it has something to do with load I think, It will go WOT in neutral with no problem. Think it would be worth trying a different PCM? Also fuel pressure drops and rises from 40psi to 30psi back 40psi in tune with the surging which is probably due to the PCM shutting down becuase ive replaced the pump regulator and filter. Not sure if that info is of any help but thought id add it.