Problem in mocking

Hi,
i m having a weird problem.i m mocking a method and when i run this test sometimes it passes and sometitmes it fails.what happens is it fails while creating a new instance of an object and gives the following error
"System.InvalidProgramException.CLR detected an invalid program".Another reason for my confusion is that the exception is coming from a different class than the one i m mocking.here is the code of my test

MakeReservationRequest request = ConstructValidNRSReservationRequest("O",1);
MakeReservationResponse response = new MakeReservationResponse();
Mock reservationController = MockManager.Mock(typeof(ReservationValidator));
Mock seatReservationController = MockManager.Mock(typeof(SeatReservationController));
Mock reservationServiceAgent = MockManager.Mock(typeof(ReservationServiceAgent));
response.MessageHeader = new global::Ttl.Tissip.Messages.Common.CommonTypes.ResponseMessageHeaderType();
response.ResponseDetails = new ResponseDetails();
ReservationValidator.NRSSeatReservationStatusCode reservationCode = new ReservationValidator.NRSSeatReservationStatusCode();
reservationCode = ReservationValidator.NRSSeatReservationStatusCode.NoSeatsAvailable;
reservationController.ExpectAndReturn("SeatReservationStatusCode", reservationCode);
seatReservationController.ExpectAndReturn("ProcessReservations", true);
response = ReservationController.RequestForReservation(request);
Assert.IsTrue(true, "Test for NCover failed.");
reservationController.Clear();
seatReservationController.Clear();
reservationServiceAgent.Clear();
reservationController.Verify();
seatReservationController.Verify();
reservationServiceAgent.Verify();
MockManager.Verify();
thank you