[Dmitry at Pelican Parts]

Sounds like the motor/regulator need replacing. Take a look at the link below to our site's DIY tech article on this procedure. Please let us know if you have any questions!

Porsche 911 Window Regulator and Motor Replacement | 911 (1965-89) - 930 Turbo (1975-89) | Pelican Parts DIY Maintenance Article