My truck runs good. Put some times shuts off like the idle is to low. So I turn the heater fan on and it idle up. But run rought. After awhile it starts to run smoother but at high idle. So turn off the heater fan. And the idle is back to 800 / 1000. I checked the fuel filter and fuel line.N the fuel pump is pumping. I looked for broken or disconneceted vac lines too.