I've remember reading on various threads that having to bang the fridge near the thermostat sometimes means its getting stuck. If you end up needing a new thermostat, I was able to get the GC403 to work while retaining the original placement and control knob - took a little ingenuity to mount but it will work.
There isn't a separate thermostat in the freezer section, the whole system cycles on and off strictly based on the refrigerator temperature so its important that the sensing tube is attached properly I learned.
I also had issues with frost build up in the refrigerator section that impacted the cycling since the frost was so close to the sensing tube. This was caused by one of the defrost heaters not operating. I was surprised at how much of a difference this made once it was fixed.
In addition I had issues with the fridge gasket which impacted the temperatures as well. Even though it looked OK and appeared to be sealing - it actually wasn't and so once I replaced that and got the door realigned things improved substantially as well. The magnets that hold the door closed do have some adjustment in them to help make the door seal better.
Hope some of my observations help you. They may not have anything to do with the problem you are experiencing but sharing just in case. I learned through the process of getting mine working properly that these fridges are a bit tricky and all the stars need to be lined up perfectly.
