After the drive as run for a while the head break will suddenly click on. Of course the drive stops responding at this point. 

I can also hear the disk RPM changing after this happens... sounds like it speeds up, then cuts the motor off for a moment, then comes back on..

Is this the Widget protecting itself due to bad RPM regulation? 

I exchanged the Servo and Controller cards from my other good drive, and the problem persisted. 

On the problem drive nothing looks visually wrong with the magnetic pickup on the motor (nothing stuck to the pickup, clearance looks about the same as my working drive)

Thanks for your input!