How much resistance does the machine have when turning by hand?
My SQ5000 motor will creep along at a stitch per minute without stalling. However, my Adler can coast through a full stitch (or even two) if the belt is removed and I "fling" the very heavy flywheel.
What is the ratio between your two pulleys? (Calculate it http://www.gizmology.net/pulleysbelts.htm
If it still bothers you after you turn up the speed a bit, you could eventually buy a smaller pulley for the motor. This would sacrifice absolute top-end speed (which most people don't need) for more low-speed torque (which everyone likes).