2001 Mustang GT runs rough when wet

Automatic, shutters when shifting down from 50mph, mechanic thinks it's the converter and added more hydrolic fluid. Probem is when it rains out the car runs aweful, it shudders and has difficulty shifting. Problem goes away when it is dry.

When sitting still, the rpms flucuate between 500-1000, back and forth (D, N, P) but when in R itsits perfectly at 7.

260-horsepower V8 engine