TypeMock 2.3.0 and TestDriven.Net

I'm running into a strange problem. I'm in the process of implementing TypeMock into my existing unit tests so i have a mixture of unit tests that use TypeMock objects and some that don't.

I'm trying to run the tests through TestDriven.Net in visual studio and i'm running into problems. when i run the tests one by one, they all pass. when i try to run a set of tests, some of them fail with the following error:

TestCase 'XXXX' failed: System.NullReferenceException : Object reference not set to an instance of an object.

The tests that fail are the ones that are not using TypeMock but the Object that is coming up null is of a type that is mocked in another unit test.

also, when i run the tests through the NUnit gui, they all pass.

Any ideas?
Michael.