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.