Sounds like you're running hot, have you confirmed total amp draw before and after breaker and incoming to bus? Just shooting from the hip it sounds like there is a bad connection at the bus or from the breaker or perhaps the contacts in the breaker...Have you checked the temp at each point? On startup the weak link should heat first and be hottest....

Found this...

From experience of Masterpact ACB's (4000A) that is a very high temp.

One problem I have had is the isolating contacts, which being between the bus-bar connections and the fixed contacts would be where I would start looking. The sprung contacts being in the fixed part of the cubical doesn't make this an easy or convenient job. The problems I had, had been caused by the contact lubrication hardening over time.

I hope this gives you somewhere to start from. Given the design of the main contacts I would be very surprised if they were giving the problem.

We had to shut our plant down to check this. The ACB was running at 160 degrees C. When we examined it, the lubrication grease, as in your case, had hardened. After clean-up it now runs at 120 degrees C. Our main concern is when we upgade the plant there will be about a 2800A draw, so the temperature I asume will rise accordingly.

http://www2.schneider-electric.com/documents/panelbuilders/en/shared/technical-ressources/mv_design_guide.pdf

this can help you. just go through the document. it mentions all the design considerations and temperature limits for Schneider ACB.

the ACB Contacts in your case is running at very very high temperature. the maximum temperature an ACB contact can handle is 105 Degrees.

what is the ambient temperature in the panel?

I have witnessed a problem with a similar breaker feeding my mainframe computers. The switch was running hot, as verified by an infrared camera. The immediate cause was the cable termination to the switch. It was still working, but the metals were oxidized and discolored thruout the whole switch - unrepairable. Bypassing it was a maior task and a pain. The original cable ends were useless due oxidization / discoloration. The new switch had less than 1/3rd of the temperature rise, and gave no trouble for many years. The postmortem told: once damaged, not repairable.

I would rather suspect the hardening of grease was a result of the high temperature. To read 160 degree Celsius is very very unusual. Areas you may like to check is the busbar cross section area, whether they are properly sized and the bus-bar contact surfaces are not having small gaps.