Server has random invisible mobs/players

My server sometimes has mobs or players become invisible (not the invisible effect). So someone will die from a random invisible creeper because they cant see it. This is becoming VERY common and its fixed by rejoining the server.
My plugins are:
Antiaura, auctionhouse, bountyhunter, clearlag, coloredsigns, craftingstore, duels, essentials, essentialschat, essentialsgeoip, essentialsspawn, essentialsprotect, essentialsxmpp, ezbroadcast, groupmanager, infobook, jobs, lastloginAPI, multiversecore, multiverseportals, parties, protocol Lib, signshop, silkspawners, superbvote, timthenchanter, vault, votifier, worldedit, worldgaurd, worldgaurdextraflags

Also, spawners and mob spawning are inconsistently disabled. Certain, random areas of the world has spawners disabled while others have it enabled. Certain areas have mobs unable to spawn at all. Thanks