I dont know a VX that doesn't, common problem, the gears that drive it slip.
We've had A VX wagon, VX Sedan, And WH Statesman that all did it
The VT did it aswell, you would think they would have had it fixed by the fourth time (we had VX series two)
make sure the arial is free of any bends.. dead straight, and no kinks.. give it a tiny spray of silicone spray