Did you measure the voltage during start up? That is the point when there is a high current surge and the voltage will drop. The high current will cause voltage at each point along the electrical path. It could be the contactor or a loose or dirty connection.

You should also measure the current flow. If it exceeds the rating on the plate then that is the likely root cause of the problem. It may be a sign of a failing. It is hard to tell without on site troubleshooting.