Well seems to be going ok now. It was happening whenever I hit the submit button whether new thread or just responding to a post. I'll let you know if it happens again.
That problem CAN occur given certain circumstances with vBulletin's templates, but it's not affecting anyone else that I know of, and I checked the relevant circumstances and can't find anything amiss. That suggests that the problem is a stale cookie or something in your browser. Given the information I have, though, that's inconclusive.
If you do see it again, please try the suggestions I made and let me know how it works out.