Making the assumption that you have a V6 or even an SVT, I will tell you that it is almost def the LIM (Lower Intake Manifold). They get hella dirty and eventually stick shut of are erratic when opening. There are a million topics on the Performance and Mods sections on how to clean it. It is fairly straight forward, but it does require "some" ability to turn a wrench. Check the "How-To" and Mods forums. Good luck. BTW-I had the same problem. Do yourself a favor... Get new IMRC clips for the actuator arm. They break and will cause undue stress...
