This error causes crashes, but what causes this error?

Code:

2012-05-20 12:11:41 [SEVERE] java.lang.NullPointerException 
2012-05-20 12:11:42 [SEVERE]    at net.minecraft.server.NetworkListenThread.a(NetworkListenThread.java:63)
2012-05-20 12:11:42 [SEVERE]    at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:558)
2012-05-20 12:11:42 [SEVERE]    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:450)
2012-05-20 12:11:42 [SEVERE]    at net.minecraft.server.ThreadServerApplication.run(SourceFile:492)
2012-05-20 12:11:42 [SEVERE] Unexpected exception
java.lang.NullPointerException
        at net.minecraft.server.NetworkListenThread.a(NetworkListenThread.java:63)
        at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:558)
        at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:450)
        at net.minecraft.server.ThreadServerApplication.run(SourceFile:492)
2012-05-20 12:11:56 [INFO] Read timed out
2012-05-20 12:12:49 [INFO] Read timed out
2012-05-20 12:13:02 [INFO] Connection reset
2012-05-20 12:13:38 [INFO] Read timed out
2012-05-20 12:14:26 [INFO] Read timed out
2012-05-20 12:15:59 [INFO] Read timed out

"Read Timed Out" continues on and on for 2 hours.
Memory usage is normal,

Code:

14:46:08 [INFO] Available total memory: 6135.0 MB
14:46:08 [INFO] JVM allocated memory: 4475.0 MB
14:46:08 [INFO] Free allocated memory: 2623.0 MB

Processors aren't being strained:

Code:

Cpu0  :  0.3%us,  0.7%sy,  0.0%ni, 99.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Cpu1  :  1.3%us,  0.3%sy,  0.0%ni, 98.4%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Cpu2  :  0.0%us,  0.3%sy,  0.0%ni, 99.7%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Cpu3  :  0.3%us,  0.0%sy,  0.0%ni, 99.7%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st

And disk IO isn't over the top.

There are no further errors. Does anybody know what could be causing this? It doesn't seem to be related to a plugin, since only bukkit methods are mentioned in the stack trace.