1997 ford xlt ranger 2x4 will start but won't stay running unless pumping gas. rondomly will idle for about 20 seconds [pur like a kitten] without sputtering or touching gas, then just stall out, will start back up but must keep giving it gas. ihave changed fuel filter in the undercarriage, idle control valve, wires n plugs look good.i have checked for vacum leaks but can't find any.