I would need to see a video from the rear oblique, but I suspect your knees are collapsing in. You also are coming out of your hips and lifting your chest when it gets difficult. Let me see a set at 200 for five at hip height from between 30 and 45 degrees from the side. Work hard to keep your knees in place once you are halfway down and drive your butt up all the way to the top. Fight the tendency to let your knees shift forward about halfway up on the ascent while you lift the chest.