Two things come to mind depending on make and model...If it only does it when it's wet, does it start harder? If so, look at your distributor cap. There may be small cracks that allow moisture in. Look for carbon tracks and moisture beads in the cap. It will make it seem like it might be slipping when it's actually misfiring.

Number two, does it have some kind of traction/stability control system? If so, the ECU could be cutting in because it thinks it detects wheel spin.