Scrolling through some Thumbs -help!

ok I’m stumped after 4 days of bashing my head against the keyboard, I need your help…

I have a series of thumbnails in a masked MC and I want them to scroll through horizontaly, but only want the scroll to occur when the user reaches the edge (ie every 5th image) I’m sure it’s very easy but I can’t for the life of me think of what I need to do…

help?