idle on ranger changing

hey does anyone know or have any ideas on why my idle would be variating between like 2000 and like 500. some times when i start my truck it will idle at 2 grand witch sounds really high then other times i will get into my truck an it will be normail at like a grand. then other times i will be driving and get to a stop sign and my truck will idle and almost die at 500 rpm's. if anyone has any input it would help. o yea my truck is a 96 ford range with the four banger in it.