Top shelf of old sub-zero refigerator 60 degrees, bottom shelf OK

I would guess that this sub-zero is from the 80's. When we bought the house there was a hard start on the refrigerator. We replaced the condenser in 2001. Recently we noticed that the top shelf was 60 degrees. We lowered the temperature and blew out the condenser area. No improvement. The bottom shelf is 39 degrees and the freezer is fine. I don't know whether it's worth it to throw any more money at this vintage frig. It's built in and we don't want to spend the $$$ to replace it with another sub-zero. If the problem could be fixed for no more than $200 and it could perhaps last another 6 months to a year, it would be worth it to delay the hassle of pulling it out. Any ideas?