What's the material? How fast are you running the spindle? How fast are you feeding?
When I first got my machine, I ran all cutters at 5,000 RPM and I couldn't get a decent finish to save my soul. Then one day I decided to call tech support to find out why it was doing that. I spoke with Greg and he told me it's just a harmonic in the spindle. Slow down to 4,500 and it should get better. It did, and now if I want a good finish, I run my finish pass at 4,000 RPM. It doesn't seem to matter how fast I feed, just the RPM. It seems that I can feed 8 IPM for every 1,000 RPM I run. But that depends on the size of the cutter too.