There is no way to vary the speed on a AC single phase motor, if it slows down to 80% of full load speed the start winding kicks in and will burn the motor up. If you had a DC motor there are lots of ways to vary speed but not torque so it would seem to run out of power at lower speeds. Also if you switch between high and low speed as mentioned in the above post I would use a double pole swwitch. On off on, three position or Hi off low. If you use a double pole 3 position switch (toggle) you can jumper the poles together and double the current carring capacity o the switch. Power to the middle 2 terminals Hi to one outer pair of terminal and low to the other pair.