Something causing invincibility..

Howdy, I develop for a server, and we've come across a pretty big and annoying bug. Invincibility. Players claim they walk out of spawn, and it happens. Others claim when they die their health bar isn't going back up. I thought it was autorespawning, which I've removed from the server and it's still happening.
Plugins list


We are still completely unknowing of what is causing this. Maybe someone else is having this problem?
One of our fixes was deleting the <player>.dat file and making them a whole new player, and that seems to not work after the first time we did it. Please can someone help

http://forums.bukkit.org/threads/list-causing-invincibility-bug.178302/