Actionscript Vertical Movement Problem

Good afternoon everyone

I have been working on a website now for the past few days and have come up against a bit of a problem.

I have written some code to move an MC along a line and then down another line, as you can see in the attached file, however it does not exactly follow the line.

In the real website I have two lines, one on either side of the mc and I would like it to move exactly down the middle however one in four loops it moves slightly to the right and just looks odd.

Any help fixing this issue really would be a massive help as I am now a bit stuck as to putting the site live.

Cheers Cameron