Speed Control Surging or Hunting

[Bruners4]

I have a little problem with the factory Speed Control on my '91 F-150. It has been working correctly in the past but today it has started to act up. I was driving in heavy rain and wind while driving down the hi-way. I turned on the speed control and set the speed. I could feel the accelerator pedal moving up and down as though the speed control was hunting. Taking my foot away from the accelerator pedal, I could still feel the speed control hunting or surging. This also happened once before after driving through deep puddles of water. It cleared up last time after everything dried out. Any ideas out there?