I have a 22 year old Sub-Zero refrigerator in my new home. Over the last couple of weeks I noticed the temperature gradually increasing. The condenser had quite a bit of dust on it and I thoroughly cleaned it. It is working slightly better however temperature of the items on the top shelf are about 48° while on the bottom shelf it's 36 degrees. This is with the fridge set to 7. All the items in general feel colder than what they were before I clean the condenser but is this fridge supposed to have that kind of variance?