Turning slowly in high winds indicates a short circuit on the generator. The Proven turbine uses a permenant magnet generator, and shorting this out very effectively brakes it. Even in very high winds, the turbine wont turn more than 2 or 3 rpm
The short circuit can be in 2 forms - External or internal. External short circuits are easier to deal with. If you switch off the turbine controller isolator, and the turbine suddenly starts up, the short circuit is downstream of the isolator, possibly in the controller itself or the inverter.
If the turbine does not speed up when this isolator is opened, the short circuit is either in the generator windings themselves (more probable) or in the cabling between the generator and the isolator (less probable). The only way to determine which, is to split the cabling at the bottom of the tower, then let the brake off and see if the turbine turns. If it still turns very slowly, the turbine has to come down.
If the fault is in the turbine, most likely it is cooked generator windings.
If the rotor is turning even with the brake on, the brake is not on tight enough. Tension in the brake rope needs to be as much as you can manage, otherwise the brake will slip. You need to pre tension the rope before winding it round the brake lever, then pull the lever down hard. This takes a lot of effort. If it doesnt feel like you are about to break the rope, then it isnt tight enough!