ControlSource is property that shows to Combo to what field of your table it must be bound. There is no need to set it over and over again during the form run (except if you want someting else of course). So set it in Designer and forget it or set it there where you want, but there is no need to be set at every Click.
I can't understand what could cause your problem but if you want you can sned me that form with some simple data to check. The I can answer you (If I could).