obviously its happening in the rain..., do u also have the lights and heater and stereo on driving in the rain? I find with all the old dirty electrical contacts the whole charge system struggles to cope under all that load.

But then mines not a daily driver and i dont take her out in the rain if i can avoid it either.

If yours is a daily driver and has been on the road for 40 odd yrs, it probably needs a good lube up of the moving parts easy to get at under the dash, and clean of contacts, then if still slow replace the wiper moto, its only 3 screws easy to get at and not worth trying to get the originan rebrushed.