When making a ripple afghan you need to be careful at the end of your rows to count your stitches. It always looks like you're supposed to make more stitches than you're actually supposed to make. I had this exact problem with my first ripple afghan and it turns out that counting right at the end of the row (after your last peak or valley) fixed it.
If the extra stitch is coming at the beginning of the rows, then you might be making your first dc into the first stitch, which you're not supposed to do if your turning chain counts as a stitch (it normally does in dc).
If it's not either of those things, then I have no idea.