You either have a worn out blower motor, a flaky bar fuse or a bad blower motor controller.

Start by REPLACING the bar fuse. It is in a small carrier in front of the master cylinder. It will look good but have a slight crack from vibration that is difficult to see.

If that doesn't work, Remove the windshield wiper motor and the grill below the w/w motor to gain access to the motor and controller. Check the voltage to the blower and see if it reacts properly. The brushes may be worn out in the motor. If you are getting full voltage and the motor is still not turning as it should the problem is probably a worn out motor or brushes.

Also see that the motor turns freely.

Good luck,