A/C dial problem

Your interior fan resistor is mounted in the heater/AC box just behind the glovebox.

You may also have a problem with your interior fan motor as well if it "slows down and speeds up erratically" or at least a bad electrical connection.

You will need to sort out this problem first.

If there is a problem with the fan motor that is causing excessive current draw and you only just replace the resistor it will more then likely just burnout again.

I would start by having a good look at the wiring down near the interior fan motor and resistor, is there any damage to the harness or melted plugs?

With the interior fan running just start moving the harness around and see if it starts playing up, especially at the plug thats connects to the interior fan motor.

Also remove the interior fan motor, does it spin freely by hand? is there excessive movement in the shaft?