what causes the fan motor to fluctuate while driving? my heater works so does the a/c

well there is a little goodie called the 'fan clutch' in front of the radiator, in fact is an additional fan...sorta, speeds up when the moto is hot, and cools down when the motor is cool. It's job is to vary with temperature.

If you are asking about the blower fan motor for the HVAC system, it means that the brushes are too short and the motor needs to be replaced. It's a very easy DIY job on most GM vehicles. Three small screws and a electrical connector.

oops-