Automatic Star Delta motors are controlled by power contactor's provided with NO VOLT coils for switching ON / OFF the contactor's. The No Volt coil are designed for operation in a permissible voltage band (either designed for Phase to Neutral or Phase to Phase) beyond which the manufacturer do not guarantee performance. No volt coils are available in normal or wide band voltage range. In your case I suspect that the voltage variation is high and beyond the range of the No Volt coils of the contactor's. That may be the reason why the contactor's are dropping off. Another reason could be the dust / rust accumulated in the core of no volt coils which may not allow the coils to hold firmly. Replace the No volt coils with wide operating voltage band No Volt coils and also carry out preventive maintenance checks on the contactors to trouble shoot.
If you want consistent performance, You may need to provide automatic voltage regulator equipment in your plant, either at main panel or at individual equipment level, to compensate for voltage fluctuations. The reasons of voltage fluctuations are too many to enumerate here. It could be wider public utility system deficiency or a localised problem.