I tried everything suggested & it helped some. Ended up switching to the open toe walking foot (no ditch guide) and it was night & day. I think that guide was creating drag & would drive me over the seam any time my pressing was not absolutely precise & perfect. With the open toe, the stitches are all perfectly even & my line is entirely in the ditch except for a couple tiny wobbles that really aren't noticeable until you're 6" from the quilt & looking for them.
Many thanks again to all of you!