[87 GT MUSTANG]

the problem with your car could be the throttle position sensor, i just had the same thing as you described wrong with my car since i bought it and i recently ripped apart the top half of the engine and cleaned it up and put it all back on and it turned out to be my throttle position sensor, but before you go pulling it off first check the adjustments, place two pins in the wires one in black being negative and the other in green and get a volt meter and check that it is at .980-.990 for best performance and if you cant adjust it to that drill or file out the holes on the tps. Hope this helps you a little