The banging problem that disappears when the door is open is most likely a fault with the main control board located at the bottom rear of the appliance.
As it's intermittent you may want to live with the problem for a while yet.
The fluctuating temperature can be 1 of 2 things.
The air diffusor on the top shelf is freezing in either the open or closed position (this is why the temperature is fluctuating so much). Replacement of this part is the more likely of the two possibilities.
The second possibility is the main pcb at the bottom rear.
The main pcb controls the entire appliance.
The best thing you can do with this appliance is, while it's still working, put it on a repair contract with the likes of Domestic and General , British Gas or anyone else that does such a thing.
The cost of parts are very expensive and my own machine, which is now 7 years old, has had several main control boards (pcb), 3 front boards, 4 ice makers and one ice dispenser motor, I've lost count of the air diffusors and I'm glad it was on contract with British Gas.
If you want to carry out the repairs yourself the first thing to do is strip out the fridge plastics to expose the air diffusor located on the top shelf left hand wall.
Once exposed you'll be able to tell if it's frozen up.
If it is, you can try taking it out (you have to strip out the freezer to gain access to the rear of the diffusor to release it) and thoroughly drying it out with a hair dryer. Refit it and see how it goes.
Hope this helps
Let me know how it goes, what you find or if you need more help.
Yes, it's a Whirlpool. I used to work for them and know the problems well.
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It will take you a while to read through the posts on the forum but it will give you some idea of how common the pcb problem is.
Good luck chasing the supplier and credit card company. Hopefully the forum posts will be of some assistance to you.
Sorry, I can't help with the companies as they will all want to repair the appliance.
I actually think getting Whirlpool out to diagnose and quote is the best idea.
They make the appliance and should be able to diagnose the fault correctly.
Who better to give you the information in the battle?
Good luck with it.