Is it possible to execute unit tests with dotCover class after class?

Hi

In my unit tests I have to manage with a singleton sometimes, I'm affraid that Unit Tests tool of ReSharper creates several threads to execute unit tests, this causes that some test fails due to concurrent access to my singleton class but if they are execute individualy by class, they work perfectly. Is it possible to execute my tests class by class? I don't care about the extra time, but now I have a not reliable coverage.

Thanks a lot for your help