If you go to the air handler and open the blower compartment and smell the open end of the motor, does it smell like burning rubber? It's possible the windings are slowly shorting but you'd need to take the motor to a shop to know for sure.
With power off, spin the blower wheel (squirrel cage). Does is spin freely and quietly? You might run the blower with the door open to look and listen while it runs. There should be a switch at the door opening you'll have to press manually. Possibly the motor bearings slowly bind as it runs. You should be able to hear that problem.
Another possibility is the run capacitor is failing (assuming a PSC motor). It's not expensive and easy to replace. I'd start there given the age of the system. Be sure to buy an IDENTICAL capacitor... take the old one with you.