the reason i say the pump,is that when driving along,the pump is flowing quite freely,pluss the vac pipe to the tank will give some pressure,to boost fuel flow,if your pump is on its way out,the impeller could be slipping causing erratic fuel flow whilst under little pressure is supporting the feed,like mine done,
BUT HEY! (in an american/bronx/italian/goodfellas accent)

"im juss a fookin painta,who doesnt give a shyit!" " know warra mean?"