Server shuts itself down

Specs: 1gb ram, 2gb burst.
CentOS x64 5.5
openjdk 1.6.0

It seems to be based on the amount of RAM, because when I limit it to less, it crashes faster, and when I let it use as much as it wants, it lasts a few minutes longer.

It also just shuts down, the process stops running, no errors starting up or shutting down, even with vanilla minecraft.

I personally went through the process list through ps aux and killed the apache and mysql ones that seemed to be sucking up memory.

Also gets this error:
[root@** User]# cat nohup.out

Exception in thread "main" java.lang.NoClassDefFoundError: com/mojang/minecraft/server/MinecraftServer
Caused by: java.lang.ClassNotFoundException: com.mojang.minecraft.server.MinecraftServer
at java.net.URLClassLoader$1.run(URLClassLoader.java:217)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294)
at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
Could not find the main class: com.mojang.minecraft.server.MinecraftServer. Program will exit.

Does anyone know the solution to this?