Petr
I'm presuming your mechanic took away the plastic disc in the roof lining, and adjusted the clutch?
It is commonly accepted that the motor gets "tired" with age, and the toughest part of its job is lifting the roof up from its fairly tight fit. If this is the case, then the cure would be either replacing the motor, or taking it apart and refurbishing it, lubrication, new brushes and so on. It strikes me that this would require taking the sunroof apart, and the roof lining, maybe more trouble than it is worth, to get at the motor.
Recently I have read a thread in the USA. Beav, who knows his stuff, says that the packing material around the roof swells with wet weather, making the roof a tight fit, making it stick. His cure is to clean it thoroughly, and to use plenty of lubricant to make it easy to shift upwards. This seems to work for him.
If it works for you, it is a solution. Otherwise, I would keep it lubricated and give it a nudge to kick up at the start. Unless you really really want to take it all apart?
Joe
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