For me, though, when that kinda stuff happens my first response is usually to try dropping the spindle RPM.

Increasing feed power and speed, decreasing spindle RPM, using a smaller diameter endmill or one with fewer flutes will all increase chip load. Are the chips clearing properly? If the feed rate is correct but the chips can't evacuate that'll cause problems too - switching to a different toolpath strategy (eg deep and light instead of 2/3 diameter and shallow passes) might make enough difference.