Ok those drives usually come with a fixed field supply built in, so as the motor heats up your field current slowly goes down loosing torque so the drive has to put out more Armature power to do the same work.
Does the motor have a cooling fan on it?
If it does make sure it is running, and if it is a Squirrel Cage (sometimes called centrifugal) type make sure it is spinning the right direction. That type of blower blows air in the same direction if it runs backwards or forwards, big difference in air flow though if backwards.
Yah actually it is funny you mention that cuz 1 of our old employee’s (Freddy, retired now) showed up a little while ago and said the same thing. Turns out it is running the right way but the inlet filter is so bad with dust it wasnt doing much. good call John