It should have been fine.
The cap should have helped a lot too.
The "twitching" makes me thing there was a problem with the transformer, a diode, or the motor.
I am pretty sure with motors that size (0.58A), the PWM isn't worth the effort.
At best, the fan above saved a hair under 25% at low speed. Much less savings at higher speeds.
Myself, I'll stick with simple power resistors until the motor is quite a bit more powerfull, maybe 3A?
Samoa's circuit is easy to build. Works fine with the changes I told my friend to make (Schottky Flyback, and better mosfet).
You can build a "regulated" transformer. Same as what you had, but a bigger cap.
Then feed the output to a low drop-out 7812 (TO-3 size, with heat sink), and a small cap and 1M resistor across the final output.
It is not efficient. I wouldn't do it for a motor.
Really, a "12V" motor should handle 16V no problem.
G-
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