Problem in the rain

Hello, I drive a Volkswagen Golf 3 AFN, manufactured in November 1996.
As soon as the road gets wet, I can't steer to the right or left anymore!
I can only get the steering wheel turned with a lot of force!
It seems like the servo is turning off.
'When I steer, the steering wheel feels stiff and starts to jerk, but this doesn't always happen. What could be the problem? The power steering fluid is full, maybe a little too full, but it's been like that for a while. I'm at a loss, and it's becoming dangerous.'
Best regards, Erik.

Hmm, could the serpentine belt for the power steering pump possibly get wet and then simply slip? Test the pre-tension, and if the edge is frayed, adjust or replace it.