All above suggestions are great and may work.
Have you considered putting a voltage recorder / datalogger in parallel with the contactor coil ?
This may record unusual voltage rises over certain periods of power applied to the coil, which cause the burn outs.
If the contactor bridge banks and the contacts can't seat properly an excess current is drawn through the coil which will result in a burn out, usually the contactor is noisy too under those conditions, sometimes some vaseline will help applied lightly on the moving guide parts.
Check coil neutral with an independant earth tester just in case a poor neutral exists which may shift during different unbalanced loads and cause a higher voltage to be present at the coil, although the datalogger will pick that up too.
