Solved "Killed" crashing server randomly

Hello, I'm currently hosting a server on a EC2 Micro Instance (free tier) running Ubuntu 12.04 with JRE7, and it appears to be working mostly fine with no lag.
However, the issue is that every once in a while (it ranges from an hour to 6 hours), the server randomly crashes with just a message saying:
>Killed.
I have allocated 480MB RAM to the server, and /gc reports quite a lot of free RAM.
It happens even without any plugins installed, and there are no crash reports.
Xingc is enabled in the initialization flags.
Users received a connectionreset or readtimedout error (I forgot which).
There are no logs in crash-reports relating to this.

To mitigate the effect, I have set up remotetoolkit and autosave every 10 minutes, but I would like to find out the actual cause of the problem, as this is rather annoying. The CPU usage usually hovers around 11% with 3 players on. When I set the smallest Java heap size below 384MB, the server takes a long time to initialize, so I have kept the heap size at 480. It rarely goes over, and the SWAP is relatively fast.

I have tried both Spigot and CraftBukkit (1.4.5R0.3 for Spigot and 1.4.5R0.2 for CraftBukkit), and it happens on both. I'm currently running Spigot because it seems faster and uses less memory. When RemoteToolkit is installed, the "Killed" message doesn't show, and the server just automatically restarts. I've read another thread about this issue, but no solutions have come up.

Thanks in advance.
Plugins list:

 

Fixed it by allocating a 1GB swapfile, using the swapfile results in no additional lag.