my mustang keeps sputtering out and stalling . . like 17 times in an 8 mile drive . . this is rediculous . . ive had it for a month . . i thought it might have been bad gas so i filled it up with shell and a bottle of STP . . now it feels even worse . . it will go from 1.5 to 2.5 grand on its own and i wont even touch the gas . . then it sputters between 800 and 300 rpm and dies . . the starter works really hard to turn it over again every time . . someone help me please