The first question that comes to midnd is where did the end mills come from? Are they actually end mills or engraving bits? Second are you milling with the drill or are you using a set of collets? End mills often slip in drill attachements. Another possiblity is that your quill is drifting to deep during the cut.
I cut with small enco 2 flute 1/8 and 1/16 and run at 1750 at 6 to 7 ipm and a depth of at least .025 with out any problems.