I believe the part you'r talking about is the idle actuator selenoid.It's a small round cylinder with a plunger that pushes against the throttle shaft,and has a wire connection on the other end of the cylinder.It sounds like the most likely cause of you'r problem.Before replacing it though.I would check the plug in wire connection for corrosion.If it's at all dirty or corroded,there's a chance that cleaning the wire connectors with a small wire brush untill shiny.And then putting a small amount of Vasaline on the connectors to prevent future corrosion,may be the only repair you need.If it still doesn't fix you'r idle problem.You can try unbolting the selenoid bracket and do a similar cleaning in case of grounding problems.
If you'r car still doesn't idle correctly,you probably do need to replace the idle actuator selenoid.You can test the selenoid itself by hooking jumper cables to you'r battery.And with the selenoid removed from you'r car,hook the ground clamp to the mounting bracket.And then touching the wire connector to the positive battery clamp.If the plunger moves one way with power,and the other way with no power,it's probably fine.With the power on,you shouldn't be able to move it by hand.If you can,it needs to be replaced.
If the selenoid is working properly,and you'r idle still fluctuates quite a bit.You then most likely have a vacume leak instead.I hope this helps you out.