Most likely the run capacitor is going. It's an inexpensive part that's easy to replace. Turn off the power and open the motor compartment to gain access to the motor. The capacitor will be mounted on the motor case or on the furnace frame and has 2 wires. Remove the capacitor and get an IDENTICAL replacement... same microfarad, voltage and physical shape. I would go ASAP since you don't want to burn out the motor with a bad capacitor. You may have to go to an HVAC supply house for an exact match.