If you know the refrig is otherwise fine you could look at replacing the control board.
Dinosaur Electronics has replacement boards from many models and most of their boards have temp control adjustments. They also tend to operate a bit better than the original control boards.
They also have a temp control addon that may work with your unit. Check their web site for details. They're usually pretty good at answering questions via e-mail too.
From your description and the fact it has worked well in the past I'd be concerned that maybe it has a leak and/or is low on refrigerant.
