Help! Constant server crashing and rollbacks

Hi

Yesterday my server crashed for no apparent reason, I didn't see any error in the config as well so I didn't think much of it, maybe the internet went away. Today the server crashed again, it has never happened before and this is the error I got. I can't understand what the error is about so if anyone could please at least describe what is causing it, I would very much appreciate it.
Error:

Code:

2012-02-12 18:25:31 [SEVERE] java.lang.NullPointerException
2012-02-12 18:25:35 [SEVERE]    at java.util.Hashtable.get(Unknown Source)
2012-02-12 18:25:35 [SEVERE]    at performancetweaks.bin.org.jbls.mrchasez.ChunkPersistance.run(ChunkPersistance.java:147)
2012-02-12 18:25:35 [SEVERE]    at java.lang.Thread.run(Unknown Source)
2012-02-12 18:28:26 [INFO] Connection reset
2012-02-12 18:30:26 [SEVERE] null
java.io.IOException: Input/output error
    at java.io.FileInputStream.readBytes(Native Method)
    at java.io.FileInputStream.read(Unknown Source)
    at java.io.BufferedInputStream.fill(Unknown Source)
    at java.io.BufferedInputStream.read(Unknown Source)
    at jline.Terminal.readCharacter(Terminal.java:99)
    at jline.UnixTerminal.readVirtualKey(UnixTerminal.java:128)
    at jline.ConsoleReader.readVirtualKey(ConsoleReader.java:1453)
    at jline.ConsoleReader.readBinding(ConsoleReader.java:654)
    at jline.ConsoleReader.readLine(ConsoleReader.java:494)
    at net.minecraft.server.ThreadCommandReader.run(ThreadCommandReader.java:31)