Mongo 3 Intermitted Crashes
Open up /var/log/messages and search for mongo.

If you see "oom-killer" or "Out of memory" then this points to your problem.

We had that problem when running a mongos and multiple mongods on the same machine, for testing purposes.

Since there is no way to limit mongo's memory usage, we just added a 4GB swapfile, so there was some leeway when memory was at its limit.  (Using swap is the recommendation here.)

More on the OOM killer here.