I had a very similar problem. The defrost system wasn’t working properly and the evaporator coils were frosted over and air was not circulating into the fridge well. After defrosting, the fridge returned to normal working order. For a while. A few months later, the problem came back. In my case, a clue was that the hole that drains the defrosted water was plugged with ice. I was slow to figure out that there was a clog of gunk in the drain line (not sure where it came from) because it never occurred to me that was possible. It seems to be working better now after defrosting AND cleaning out the blockage in the drain line.