Could anyone explain why in Premier Pro 2 the sound synchronisation seems to go out of sync of it's own accord?

This is on completed films that I have done some time ago but decided to add more sound on another track? Even thought the video and audio are still linked as they were originally?

I am also having a similar problem with new clips. Play them from the avi file and they are OK but through Premier Pro 2 some seem to go out of sync. Only had this problem in the past three months or so. I thought it might be one of the camcorders (Canon XH-A1's) but tonight I went in to re-edit a previous edited film that was OK originally, some clips are out of sync whilst adding music to another track - I am baffled unless there is some ctrl + key that does this automatically (and I don't know I am doing it) but I can't find anything in the help.

Would really appreciate some feedback, but not too technical. Many thanks, John

is fine within the clip - was fine when it was comp;leted when on occasions after being edited