Do you hear the motor running when it is "stuck"? If the motor is running it could be a stripped gear. I would remove the motor and then open up the back and inspect inside. If it is a mechanical issue you may see the stripped gears. If it is electrical you may be able to replicate it while on the bench and see what is going on.
Also, if when it gets "stuck", and the motor is not running at those times it could be a loose connection. There are two circuits to the motor, one to switch it on and off and the other is for park position. I would be surprised if both connections were loose, but stranger things have happened. If the switch connection were loose it would still return to park. If park were loose when you turned the wiper off it would stay in the position it was at when you switched it off.
And then there is always Rain-X 😀
