In-Tank fuel pumps have a high failure rate on these! we have changed 2 in the last six months, both with the same symptoms as you are getting.You may hear the pump running in the tank, but is it supplying enough pressure and flow?If it changes tune as you say, then I would first change the fuel filter and if this fails,check all fuel lines.After that it will mean a good test of pressure and flow from the pump! HTH you out, Gunner.