Problem with Roof error on S4 Cabriolet.

[c3kas]

On my S4 B6 Cabriolet I keep getting a roof opening error. As the button is pressed the roof starts to open and raises about 200mm and stops. The dash shows an error by flashing the roof lifting symbol. The roof will neither raise or lower at this point and hence the roof and the rear boot flap is open. If I close the roof manually by means of the special tool supplied, everything is secure. I can then drive the car half a mile and stop and the roof operates fine. I can then use the roof half a dozen times plus and all is fine. Then it will stop again...............
ANY THOUGHTS ON HOW TO RESOLVE THIS PLEASE....................