the "switch" is simply a cable operated slider, if the knob turns the only problem would have to be in the slider on the wiper motor, could even be the cable clamp is loose .... vacuum wipers will always slow down on acceleration, because there's a lack of vacuum to run them - with the engine running poorly I'd suggest you focus on that, you need proper vacuum first, until it's running and driving well don't worry about the rain
Bill
