Why does my car keep stalling out?

I recently bought a 1987 Mustang and it keeps stalling out. It starts just fine but when it idles it starts to sputter and acts like it's going to stall out. When Im at a stop sign or something it will stall out when I hit the gas. It also stalls out when I barely tap the gas...like to drive slowly. It sounds almost like it idles too slowly. Someone told us it was the idle control valve but we changed it and it still does the same thing. Do you have any idea what this could be?