Variable success in running unit tests - Node, Mocha, Chai, Karma, Grunt

I have a test suite that I've built around my JavaScript application code containing ~350 tests. My tests are run by running grunt test from the node command line. Each time the tests run, a different amount of my tests run before it stops. It rarely finishes all of my tests. Has anyone had this kind of thing happen before? Any thoughts on what could cause it?

When I run the tests, I usually get a seemingly random number from 200 - 350 test completed. After the test, I don't get any errors or anything. It ends saying something like:

Finished in 1.332 secs / 0.823 secs

Summary:
√ 311 test completed