It's important to note that the horizontal hold control is not with the rest of the controls at the bottom left of the monitor (looking at it from the rear). It's in a weird place on the right, above the two controls there. It's in the middle of that board, facing to the left. You've got to adjust it with a screwdriver.
Or at least, that's what my DK cab is like. I have major horizontal hold issues too, but tweaking that thing will keep it together for a while.
But, like everyone above me has said, just cap it.