There could be many causes. Often it's caused by the starter
"chattering" or "plugging". Have you had an opportunity to observe how often the starter pulls in over a given time period ? Is it possible a switch or thermostat is causing the starter to pull in repeatedly ?
All six contacts, or only one phase's? Did the compressor funtion correctly at first?The contacts on one of the contactors that energize a compressor for our new Chiller is all burnt up. What typically causes a new contactor to do that? I can understand why a old one would do that.