TestNG Tests continue forever (but not always)

My TestNG tests are weird... sometimes they finish, but sometimes they don't, even though there is nothing random.

Most of the tests run, then nothing happens, except the TestNG result bar is green, and flickering every few seconds (as if something would happen in the background)

It's not always at the same test method.

The problem occures about every 3. run.

btw., there is no while-loop or something similar in my test classes and tested classes.

I would be really thankful if someone has a solution for my problem. Thanks, Thomas