I would guess you have a 12 volt
battery and charger issue and that your batteries are not charged enough to run the furnace properly. So, it may not be the furnace at all.
In addition, if it is the original furnace, it is 41 years old now, and I can only say it needs replacement due to age. The Furnaces installed in the 70's were all subject to recall due to some dangerous issues and at this point could have developed even more problems such as rusted out combustion chambers.
So, electrical problems need to be resolved first, then strongly consider furnace replacement. Even a new furnace will not run properly with bad batteries and converter/charger issues.