Net frequency error.
1. Is power supply between 59 and 61 Hz?
Yes: -> 2.
No: Provide frequency stabilized power supply rated at 60 Hz +/- 2%
2. Is power supply circuit breaker loose or faulty?
Yes: Replace circuit breaker.
No: -> 3
3. Is field wiring loose or faulty?
Yes: Correct wiring.
No: Replace Control Module.