I see your problem and it is simple. You are not allocating enough RAM to the server. Your solution would be to get a free software if you haven't called notepad++. I highly recomend it to any server owner; however, you can also use regular notepad. Next, open up your file that contains your server's folders. Open up notepad++ (or notepad) and create a file with this in it.
"%ProgramFiles%\Java\jre7\bin\java.exe" -Xmx3060M -Xms3060M -jar craftbukkit-1.5.2-R0.1.jar
IT MUST HAVE THE CORRECT SPACING!
save it as run.bat
MAKE SHURE IT IS A .bat FILE
close out your server and run the newly made .bat file and u should be fine. WARNING: this is for servers with 4GB of ram or higher. This is also only compatible with Windows and uses Java.