Game freezes / crashes at random times.

What happens: I'll be playing on a world or a server and it will randomly crash, even when I'm not doing anything but placing blocks.

When it crashes Minecraft freezes and to do anything I have to restart my computer.

So far this has happened on a couple of worlds:

• An online marketplace map (Infinity Skyblock by Jigarbov)
• The same map, but offline
• An offline creative world
• The hive server

It usually takes around 20 to 30 minutes before it crashes, it never crashes right away.