Hello John
I have a very similar problem and at least part of the trouble appears to be ventilation at the back of the fridge. The space behind my fridge got pretty warm and I thought the high temperature wouldn't be helping matters. I fitted a 12V fan to blow the air through and this worked so well my fridge became a freezer. I fitted an extra ventilation grill in the floor below the fridge and that seems to allow the fridge to work most of the time - on very warm days I still use the fan.
I am wondering though, if it is possible to replace the ammonia solution as I have a suspicion that over the years the ammonia part escapes.
I hope this helps.
Paul, April 2012