thats not a very agressive cut at all. ive done similar cuts in 304 stainless at 20ipm on the nm200. i can do the cut you mentioned on my nm-070 (with poor results due to bad z gibs).
that cut should only be exerting 20lbs of force, so the first thing id check is if i could see any play pushing on the head by hand. if you get slop, perhaps the bearings arent preloaded correctly - or theyve worn prematurely.
if its chatter thats inducing this error, try to adjust the feeds and speeds to his a sweet spot.