My server keeps going "Read time out"

When my server has been on for a while it says "Read time out" in console and none can join.
the machine hosting my server is a windows server 2008 x64 STD, java version is latest 7.
and the server has 1.5GB ram. Im using bukkit RB 1337

This is what I find in console except "Read time out"

Code:

2011-11-06 13:51:11 [SEVERE] java.util.ConcurrentModificationException
2011-11-06 13:51:11 [SEVERE]     at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
2011-11-06 13:51:11 [SEVERE]     at java.util.ArrayList$Itr.next(Unknown Source)
2011-11-06 13:51:11 [SEVERE]     at net.minecraft.server.World.tickEntities(World.java:1109)
2011-11-06 13:51:11 [SEVERE]     at net.minecraft.server.MinecraftServer.h(MinecraftServer.java:467)
2011-11-06 13:51:11 [SEVERE]     at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:374)
2011-11-06 13:51:11 [SEVERE]     at net.minecraft.server.ThreadServerApplication.run(SourceFile:417)
2011-11-06 13:51:11 [SEVERE] Unexpected exception
java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
    at java.util.ArrayList$Itr.next(Unknown Source)
    at net.minecraft.server.World.tickEntities(World.java:1109)
    at net.minecraft.server.MinecraftServer.h(MinecraftServer.java:467)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:374)
    at net.minecraft.server.ThreadServerApplication.run(SourceFile:417)

 

Don't make multiple threads.