A sheet processing apparatus comprises at least one roller disposed in a sheet conveyance path and configured to convey a plurality of sheets; a sheet sensor disposed in the sheet conveyance path and configured to detect each sheet as it is conveyed by a particular roller of the at least one roller and to generate sensor information indicative of the detection; and a controller configured to: use the sensor information to calculate a sheet passage interval for each sheet; count a first number of times the sheet passage interval is greater than a first threshold corresponding to abnormally slow sheet conveyance and a second number of times the sheet passage interval is lower than a second threshold corresponding to abnormally fast sheet conveyance; determine whether an alert condition has been met based on the counts of the first and second numbers of times; and generate a control signal after the alert condition has been met.