Why is it that a 98 Bonneville will bog or lose power while driving on the highway only when it is raining heavily and then all of a sudden drive fine?

Why is it that a 98 Bonneville will bog or lose power while driving on the highway only when it is raining heavily and then all of a sudden drive fine?
2 reasons - 1) Water on the roadway creates a significant drag on the tires & 2) High relative humidity in the air going into the engine slows the combustion process down.