Diagnosis
The turntable motor is failing or the drive hub is stripped. Intermittent operation is a classic symptom.
How to Fix It
Remove the glass tray and roller ring. Inspect the plastic drive hub in the center of the cavity floor. If the square hole is rounded out or cracked, replace the hub. Part number is usually 6323ER2002A.
If That Didn't Work
The turntable motor itself is faulty. You can test by turning the hub by hand when the microwave is running. If the motor shaft does not turn, the motor needs replacement.
Heads Up
Ensure the glass tray and roller ring are correctly seated. Misalignment can cause binding and mimic a motor failure.