My problem was not so much flexibility as bad form. I think I've narrowed it down to actually carrying the bar too low. I have short legs and a longer torso, so I end up with a more vertical back angle, and the bar tends to roll down my back if I don't have it in the right place at the start. When I keep it a little bit higher, I have much better results.

Saw your post in my log - did you try Sami's elbow warmup? It's helped me quite a bit.