The server slowly uses more and more ram over a period a week until it finally consumes all memory. Server must be reset in order to start over. The problems may have been happening before 1.5.1 but I failed to notice.

Here's the invocation: java -Xmx1500M -Xms1500M -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalPacing -XX:ParallelGCThreads=2 -XX:+AggressiveOpts -jar minecraft_server.jar nogui

500 MB is left for the OS and 500 MB is left for the ram disk. I did check both and they were within the limits.