People cant sometimes play or connect

Sometimes its like the server freezes but plugins is still running as usual but people cant connect and if they are playing while it happens its like the server stops responding.
This is when it happens.

Code:

2011-03-20 18:10:57 [SEVERE] java.util.ConcurrentModificationException
2011-03-20 18:10:57 [SEVERE]     at java.util.AbstractList$Itr.checkForComodification(Unknown Source)
2011-03-20 18:10:57 [SEVERE]     at java.util.AbstractList$Itr.next(Unknown Source)
2011-03-20 18:10:57 [SEVERE]     at java.util.AbstractCollection.removeAll(Unknown Source)
2011-03-20 18:10:57 [SEVERE]     at net.minecraft.server.Chunk.e(Chunk.java:475)
2011-03-20 18:10:57 [SEVERE]     at net.minecraft.server.ChunkProviderServer.a(ChunkProviderServer.java:220)
2011-03-20 18:10:57 [SEVERE]     at net.minecraft.server.World.g(World.java:1416)
2011-03-20 18:10:57 [SEVERE]     at net.minecraft.server.MinecraftServer.h(MinecraftServer.java:348)
2011-03-20 18:10:57 [SEVERE]     at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:272)
2011-03-20 18:10:57 [SEVERE]     at net.minecraft.server.ThreadServerApplication.run(SourceFile:366)
2011-03-20 18:10:57 [SEVERE] Unexpected exception
java.util.ConcurrentModificationException
    at java.util.AbstractList$Itr.checkForComodification(Unknown Source)
    at java.util.AbstractList$Itr.next(Unknown Source)
    at java.util.AbstractCollection.removeAll(Unknown Source)
    at net.minecraft.server.Chunk.e(Chunk.java:475)
    at net.minecraft.server.ChunkProviderServer.a(ChunkProviderServer.java:220)
    at net.minecraft.server.World.g(World.java:1416)
    at net.minecraft.server.MinecraftServer.h(MinecraftServer.java:348)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:272)
    at net.minecraft.server.ThreadServerApplication.run(SourceFile:366)
2011-03-20 18:14:37 [SEVERE] java.net.SocketException: Connection reset
2011-03-20 18:14:37 [SEVERE]     at java.net.SocketInputStream.read(Unknown Source)
2011-03-20 18:14:37 [SEVERE]     at java.net.SocketInputStream.read(Unknown Source)
2011-03-20 18:14:37 [SEVERE]     at java.io.FilterInputStream.read(Unknown Source)
2011-03-20 18:14:37 [SEVERE]     at net.minecraft.server.Packet.b(SourceFile:113)
2011-03-20 18:14:37 [SEVERE]     at net.minecraft.server.NetworkManager.f(SourceFile:155)
2011-03-20 18:14:37 [SEVERE]     at net.minecraft.server.NetworkManager.c(SourceFile:9)
2011-03-20 18:14:37 [SEVERE]     at net.minecraft.server.NetworkReaderThread.run(SourceFile:62)

 

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It's a known problem it seems:
http://forums.bukkit.org/search/723344/?q=Connection+Reset&o=date&c[node]=6+19