2000 1.4 Liberte Megane chugs badly in wet/damp weather?

Have had this problem for some time, had it in the garage but they didnt fix the problem. Put a new air filter and new leads on it but problem still there. When I say chug, i mean really chugs, sometimes have to go into 3rd gear to keep it going.
Its as if it is holding back even though accelerator to the board. Any ideas out there? Id be really grateful.