Automatic transmition

Hi.
I have a erratic problem with 3rd gear changing up to early with a feel of clutch slip. This problem is not always present when it is not it goes through the gears faultlessly.I have no info on history as I only just purchased the car the service book only has 2 service stamps. Atf checked following hand book instructions oil level is ok so I have decided to change atf oil to see if this solves my problem I am using Fuchs 4400 can you tell me if there is a auto transmission filter I need to change if so where can I purchase a replacement. Amica reg w366 ppf year 2000
cheers ken