This seems to be a client-server out of sync position issue.
Try jumping right after you get into slam position. Jumping forces a server position update which may solve the issue.
Thanks kami! *bearhug* I will try that next week. After one shotting, all bosses, and spending two hours at Brontes because of the above, I am taking a break from raiding this week because this issue I had last night sucked the fun out for me. We spent there two annoying hours in which I had issues, the other tank did not, he had issues, I did not, then we both had issues. We were both shocked because we usually one shot Brontes and live happily ever after.