Need movie clip to move up and down

Hi everyone,

I have a movieclip on the main timeline and would like it to move very slowly up and down on a continuous basis. I’d like to use actionscript to do this instead of tweening on the timeline. The only actionscript I can think of that might work is using the _y property but I don’t know how to construct the full actionscript to make it work. Can someone help out?

Thanks for any help offered.