Single phase motors cannot be successfully controlled over any great speed range as you have found. This is because as the AC switches, the armature needs more inertia to carry it over to the next (other) field pole. The armature slows down and might not make the next pole before the field reverses again. You could put a fly wheel on the motor, this should make the low speed more controllable BUT it will accelerate even more slowly. The best option is a three phase motor, this will "cog" around at virtually zero speed with a suitable controller and go to X 10 label speed (unless the armature disintegrates first !). For a 100W motor, use a DC one ( from an electric drill?) with a proper speed sensor and controller. The only downside is that the motor will use brushes so will only run for a few thousand hours before the brushes need replacing.
Frank