a/c is only working when thermostat is set to 63

Is it an electronic or a mercury thermostat?

You could disconnect it and connect the Y wire - that's the usual designation for the cooling call wire, and it's usually yellow - to the R wire (24V) and see if the compressor turns on reliably. If it does, the thermostat is the problem. If it's still does not (I doubt it), there's something else but at least you've narrowed down your issues. Just be sure to connect and leave it connected for 3-5 minutes and if disconnecting manually, don't connect Y to R back again in 5-10 minutes. The compressor does not like to be switched ON and OFF momentarily.