How often do you do abs?
How how often do you train lower back?
Stability issues is usually a core weakness. I would hammer your abs and lower back like crazy. If you have issues unracking 500 lbs, you should tread lightly adding in gear to play it safe (especially in that rack and especially walking it out). You could also try some of the stability work that they are doing at Lexen Extreme. Hanging chains or weight from the bar so that it is not touching the floor. Just start light and build up. Also, its hard to tell from your grainy Zapruder film lifting vids
but are you staying tight? Flex the lats and upper back, fill the diaphragm with air, unrack, walk it out and take another deep breath and squat.