While I am, unfortunately, not privy to the actual circuits used in most telescope and focus drives, I suspect that the motors which behave erratically have speed controlled by simply inserting a resistor. This is the wrong way to do it. Manufacturers do not make it easy to analyze their designs. I can get the circuit for almost any television, amplifier or other piece of electronic equipment made anywhere in the world. But some manufacturers are simply not cooperative in these matters.
Reverse engineering is required and it is very time consuming.
Small DC motors, like other motors, are not described in a very uniform way by their manufacturers. In many motor catalogs, only the rated voltage and a high speed limit are given. In some cases the power consumption is given and in some the current is given and in some the torque is given. The specifications are a hodge-podge of information much of it useless. So design is a tricky business.