Since one of the later 1.6 snapshots (I cannot recall which), minecraft servers have been unusable for me. The server starts up fine, but then over time the server gets unstable and the log just fills with cannot keep up messages. Timeouts begin to occur all the time and block lag occurs. This isn't a hardware issue, because the servers worked with 1.5 and lower with 20+ people on no problem. Now the server becomes unstable with no one on. I can however, host more than 20 people by just hosting it as a LAN world then giving my friends that port and forwarding it through the router. That results in no lag or performance degrading. RAM on server set to 6gb and computer has 16gb.