Test failes and then not and then failes again

Hi,

[TestMethod]
Isolated]
public void JOBMANAGER_fires_a_CurrentJobfinished_event_when_a_JobFinished_event_is_triggerd_by_a_job()
{
//arrange
IJob job = new Job();
IJobManager manager = new JobManager("ignore", "ignore", "ingnore", null);
// Isolate.NonPublic.WhenCalled(manager, "Start").CallOriginal();
// Isolate.NonPublic.WhenCalled(manager, "OnCurrentJobFinished").CallOriginal();
// Isolate.NonPublic.WhenCalled(manager, "JobFinishedHandler").CallOriginal();
Isolate.NonPublic.WhenCalled(manager, "Impersonate").IgnoreCall();

bool eventTiggered = false;
manager.CurrentJobFinished += delegate
{
eventTiggered = true;
};

//act
manager.AddJob(job);
manager.Start();
// Isolate.Invoke.Event(() => job.JobFinished += null);

//assert
Assert.IsTrue(eventTiggered, "CurrentJobFinished event shoukd be triggered when a job finished");
}

This test failes and 1 sec later it works..
it fails on the assert method not on a internal method of job or jobmanager