Restricting movieclip movement along a path

Hi,

I know how to do motion guides, but now I also want to control this movieclip with actionscript code, so that I modify the x and y properties, but STILL restrict the position of the movieclip only to that motion guide.

I see that once I modify the x or y properties, the movieclip jumps off the motion guide. Any ideas how to keep that movieclip on the motion guide??