Shoot it supported under the frame on a rest with no forend on it, if the vertical stringing disappears, you need to work with the forend. Try an O-ring on the forend stud, or better yet, put a 1" square of fridge magnet in the forend about an inch from the muzzle end, tighten the forend screw to 24-30 inch/lbs, the forend pressure point has worked very well for me on a bunch of Handis!
The forend shouldn't be too tight either, it should just keep the action snug when it's open, no felt wiggle, flat sand the flat side of the spacer to reduce tension on the barrel.
Tim