Javascript tests: Inconsistent number of tests

Apologies if this is somewhat vague (and it is), but I've inherited a bit of a mess from a departed contractor, and need some pointers...

I've got (inherited) a number of JavaScript Unit tests (approx. 770+). These are being run with Chutzpah (both in Visual Studio 2015, and from the command line during our Team City build).

The problem is, each time I run the tests - either in VS, or from the command line - the total number of tests run differs, as does the number of failures

It's the inconsistency that's the problem. I can't start investigating actual problems, as I don't even know if I can trust the tests that have been run.

Apologies, I appreciate I've not given any real detail here, but can anyone suggest any high level things I should look into in this sort of scenario ?

Cheers