Now looking at the videos of my deads i see exactly the issue with my strength and technique. In the video on 26/10/2012 where I tried 500, if I pause exactly at the moment I begin to apply force on my 405 warmup, you can see my upper back is rounded, putting the scapulae directly above the bar with the hips in proper position. this slight horizontal shortening of the spine allows the hips to drive through the start and stay in position throughout the pull.
if i pause in the same spot on 10/12/2012 you can clearly see that i'm holding my upper back straight, thus lengthening the distance from hips to scapulae and making it nearly impossible to keep a good position, by the time my hips rise to the point where I've got some leverage, I've been thrown forward and the pull is a mess. I will be testing this theory on Thursday or as soon as I can get to the gym.