Thermostat question

Can you provide the part number and brand of the pusher fan? I can kinda see part of it in your avatar pix.) There are a lot of cheap junk fans out there that cannot flow any real useful amount of air through the restriction of a 3 row rad. Their flow ratings are for free flow, and their airflow drops like a rock when you put them up to a rad surface.

Since you are not cooling well a low speeds and stopped, you have an air or coolant flow issue. Are you using a stock water pump?

And did you report what width Champion that you have? I don't see if it is a 22" or 26". But that is not the likely problem if the overheating is just at idle.