Need to know what "it" is.
What causes a car to idle good and run rough when you give it gas?
Could be the TPS, it will cause it to idle high and low and make the car become sluggish, but a vehicle will run without one, but it will run very rough.
many different things but basics are incorrect air incorrect fuel or incorrect fire
Your 2000 4.8 liter Sierra might run rough at an idle because of clogged fuel injectors or a clogged fuel filter. You might also have gotten water in the gasoline.
What causes my Mazda mx3 to turn off while driving down the road.
Check the Park Avenue EGR valve for a clog. A clogged valve can cause a rough idle and stall at a light.
Vehicles will often run rough when it is wet if the cap and rotor are not in good condition. Other causes are: condensation in the gas tank, improper carb tuning, needs a tune up
Run a compression test.
Bad oxygen sensor
More than likely... Tune up, or vacuum leak
If you have a worn timing chain that hasnt been changed before, that can cause the timing to jump and cause your car to run very rough or not run at all
sounds like a stuck fuel injector