Automatic Gearbox Fault

Hi all. My problem started in January this year. Driving along and Automatic Gearbox Fault appeared on my dash, but drove fine. Happened every day, usually only once. Sometimes there would be a jolt, but at others apart from the dash telling me, I'd be none the wiser. Seemed to be worse in damp/wet weather. Then for the few weeks in the summer it ran fine. Along came colder weather and the problem re-appeared. AGF comes up on every journey now....sometimes jolting, but you can hear it struggling to change gear. From once it happens, the power seems to drop, ie: crap acceleration. Also, I drop my Son off at school, and when he opens the door to get out, AGF again shows up on my dash.......and sometimes the rear windscreen wiper goes off at the same time???? It has been on a diagnostics 3 times - Nothing!! Just wish I knew what was wrong. Driving me mad. Thanks