The accuracy of the test depends which test and the local expertise in the test. In ladies with large breasts a dobutamine echo is a very good test and reduces the imaging artifacts.
I believe the nuclear stress test are said to have up to a 10% margin of error.
If you are concerned, ask your cardiologist about doing a 64 slice CT of the Coronaries or a Cardiac Cath. Those procedures could provide more definitive answers than a
stress test -- in light of continuing symptoms, it would be worth it to put your mind to
rest.