Bukkitserver on Ubuntu need help

Definitly i cant be the RAM or the CPU, i tried starting the Server with 2048M RAm, he got sucked but using free -m and top showed me that 500M Ram and 1.1% Cpu are in Usage so i dont think it is any hardware related.

I also tried switching the Java version, but it is the same with every. I tried openJDK 6,7 and Oracle 7 (didnt manged to install Oracle6). At the first start with openJDK 6 it was successful and the server generated the whole map and i was able to connect, but the server lagged heavy and doesnt load any other chunk . Console printed System overloaded... , then it crashed. After that i looked into the server.log but the log ended at 58% of the map generation.