IT systems should be to capture the time for service to a customer order across peak & non-peak hours. Any big deviations from the mean service time can be studied to check if it is an employee effect or process effect or system effect.
Ensure atleast 30 hours of training in quality improvement is given to employees who closely interact with customers. Atleast 2.5% of the annual revenue should be spent towards this initiative. There can also be on-line training tools. Recognition of good performance is also necessary since happy staff will eventually lead to happy customers.
Have surprise quality checks carried out by quality inspectors with suitable warnings that can also result in change in the restaurant management in the quality compliance fails inspection.