1999 Volkswagen Cabrio PROBLEM IN THE RAIN

My wife has a problem during wet weather when driving her VW. She says the steering seems to "lock" or become very stiff. The problems never occurs during dry days, but only when raining or roads are wet after heavy rains. A local garage said that the belt may be slipping when it gets wet, and that could affect steering. I hit it with belt dressing, but it did not seem to help much. Could a new belt be the solution. We had a 95 VW, but never had this issue.

Any ideas would be appreciated. She loves the car, but is getting frustrated with it. And me!