To check drive would be the first choice. Also mechanical aspects: gear and shaft.

Some frequency drives can lower the speed as function of the torque. Therefore, you should check if there has been an increase in power consumption. You have also a problem in the drive speed sensor if you have.

Operational fluctuations, such as damper movements, if they open the damper for the fan when it happened, the drag sometimes will overtake the speed of the separator. Dust accumulated in separator will cause an imbalance.

Check your filter for sudden discharges and overload of separator.

Sometimes, when you increase the mill draft, the classifier has the tendency to increase its speed. To prevent this effect, all VFDs for classifiers should be equipped with what is called a Dynamic Brake module, so VFD maintains its RPMs and classifier's rotor can be controlled regardless of the mill draft. Review if your VFD has a Dynamic Brake and if it is properly configured.

If it happens on only a few occasions, then, please check with Electrical / Instrumentation (control & automation) engineers, there appears to be some problem in all probability with the VVVF (Variable Voltage Variable Frequency) control system. The designer / supplier of the VVVF system can be of much help in this issue.

Separators itself never give any fluctuation while in operation. This can happen in cases when separators get overloaded or full with material due to any blockage at that time this material will restrict the separator to rotate and speed while start reducing immediately, simultaneously you will also see that rotor load will start increasing.

Also check for any materials overloading during such incidents of speed fluctuations of the separator rotor, including any excessive wear on the separator vanes, and the top and bottom bearing lubrication system.