I was experiencing problems with chatter when boring a 3.250-in.-diameter hole, 5-in. deep in stainless steel, so I increased the feed and reduced spindle rpm. When that didn't work, I tried different inserts with various geometries, but the chatter continued. What finally worked was a process that seems too easy to be true.
During the finish pass, I moved the spindle-override switch back and forth from 100 percent to 120 percent, which broke up the operation's harmonics and eliminated the chatter. Of course, it is recommended to find the best speeds to alternate for a particular application. I have also been successful using the feed-override switch.
I don't know if there is G code or way to write a sub-program for alternating spindle speeds, but it would really help me out.