Server lags out for everyone except console

OK, let me start by saying that this is a super weird issue.
I am running a Bukkit server on an Ubuntu server machine. It is a dev build of Bukkit so that I can use 1.1. When certain people log in, it causes the server to crash. By this I mean that nobody can do anything- chat, break blocks, etc, and eventually it says "Read timed out." In McMyAdmin, the memory and processor usage flatlines. However- commands through the console still work. And it only happens when certain people login.

Specs: Pentium D Processor, 1.5 GB of DDR2, Ubuntu server 11.04.

Bukkit server is wrapped in McMyAdmin.

Plugins: SkylandsPlus, Multiverse-Core, VoxelSniper, ColoredSigns, Vault, RedstonePlugin, KickNSmite, CraftBookMechanisms, Lockette, BukkitCompat, CraftBookCircuits, WorldEdit, WorldGuard, Permissions, ChopTree, CraftBookCommon, Essentials, MultiInv, VeryImportantPeople, SuperLog, ChestShop, uHome, LWC, BookWorm, mChatSuite
Bukkit build: Craftbukkit version git-Bukkit-1.0.1-R1-91-gb81df01-b1781jnks (MC: 1.1) (Implementing API version 1.1-R1-SNAPSHOT) (I have tried others and they all have the same problem.)

 

If it only occurs when certain people connect, that pretty clearly ties it to them. Depending on their connection and their location, it may be a latency issue with those users.
I had a basement server that had a young man from Australia who loved playing there, but every time he would connect the server would be nearly unplayable due to high lag, and would eventually stop responding altogether.
There's really no easy way to rectify it, even on a dedicated connection the problem may persist, if its tied to those players.

 

I have a similiar problem it just started today. The server will be fine and running I am also on a ubuntu server with 1.1.0 and all of a sudden it will disconnect all 40-70 players online console will be fine and they can relog right away but it lags out and no one can place blocks etc... No errors nothing and everyone can be logged in and it will just randomly do this maybe 5mins 20mins or an hour. It's not a single person for me and I can't pin point it i've been stairing at the console for about 14 hours now and I can't figure it out.

Someone said it may have something to do with minecraft.net and an issue and try to play it offline mode but I don't want to deal with all the hacked clients and fake names.

 

GUYS. I know the fix. I had this EXACT problem. The players' player.dat file is corrupted. Minecraft tries to load it and it can't since it's corrupted. Try deleting the players' .dat files in the world they're in, and ta da! If you can't pinpoint the player, then delete everyone's .dat files in all the worlds, unfortunately however, this will delete their inventory.