Many of the newer Milwaukee hammer drills have a over speed governor in them, in the back of the motor by the brushes, if a lot of drill dust gets in to it, it will cause it to malfunction and slow the drill down, there is a rotating disk that has an what I think is a hall effect type device that keeps the drill from going into over speed, we have had to replace the disk once because it shattered, and now the screws that hold the brush assembly in have stripped out, so I have to get longer screws that I can put nuts on.
Milwaukee has really gone down hill, those screws just go into the plastic and strip out easily. we had an older spline drive that was much better made, and lasted for over 15 years until one of the guys dropped it from a ladder.