My car is going not going proplery after it rains?

I have a ford falcon, after it rains, when you drive, its puttering when you put your foot down, I put some injector cleaner in it that helped for a week, i took it to the mechanic they couldnt find anything wrong with it, its been very well looked after and is 2001. I had the transmission checked and serviced last year they said it was mint. How can I tell if its moisture in it??? would it still go funny after driving it for an half an hour if its water getting in somewhere? Some days its fine and other days its not. and if you watn to over take and put your foot down its not changing quick its got lag so I hope its not the transmission any ideas please