It's the Orbs. When you die all the orbs are released, and from a player that has a **** ton of EXP it will most probably lag the server, or even crash it if it is enough. I was a a friends server, I stayed on creative, everyone else on survival, every time someone died, this would happen. So I had to switch to survival and spend the next ten minutes absorbing their EXP. That's the way to fix it, as unless you force the server down with Task Manager, a normal 'stop' command won't stop the server, thus the only way to fix the lag are those two options, second is untested though, as I believe even through a force down the orbs will still be there. Stop raging, Notch will fix it eventually.