you said ball nose and surfacing. if your plunge or down feed heavy?
is your tools tight in your tool holder? if your using screw in style holders it might be time to replace the lock screw.

I usually replace the screws about every 3-6 months depending on what I am cutting and how big an endmill I am using. I retighten them about every 10-30days and if there new screws always with in 2 days to reseat.

Delw