Hesitation or slipping (?) in wet weather only.
 

2005 Mercury Monterey +100K miles. When driving at 55+mph, and only in wet/snowy cold weather, it seems like the transmission slips or hesitates ever so slightly - periodically. Seems to go away when driving at a slower speed. I'm not certain this is a transmission problem. The RPM gauge does NOT seem to change when the incidents occur. Never notice when conditions are dry. Does anyone have any thoughts or suggestions for me?