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I would take the throttle body off and clean it well- they tend to get really dirty, and at idle will not allow the correct amount of air to pass (it is a very small amount at idle, and any clogging will foul this up) This repair is free or inexpensive, just use some spray carb cleaner or brake parts cleaner and a lint free cloth, make sure to get the areas close to where the butterfly plate pivots.
Also make sure that you don't have any vacuum leaks anywhere else in the intake, like small cloth braid covered vacuum lines.
Did you try it? We have a lot of problems with misfiring and rough running, especially at low speeds or idle, that are cleared up by cleaning the throttle body well. This is just a starting point, there are many variables that can cause this problem. Again, another issue would be an intake air leak, where "unmetered" air is getting in downstream of the Mass Air Flow sensor. Both of these solutions are FREE, needing no parts to be thrown at the car.
Further diag after that point may point to the MAF itself, etc.