Thats an old unit its 40 years old. Extremely inefficient and probably has a ton of dust and dirt inside of it. I suggest that you have a licensed HVAC contractor come out and give you an estimate to replace it, it should be free. Tell them your problem and ask them to come up with a solution. As a residential HVAC contractor myself I run into stuff like this all the time, and often advise homeowners free of charge.