Tested positive on igg elisa for hsv2. then tested negative on western blot 2 years after last encounter. which test to believe ?

Western blot: The ELISA screening test is sensitive with a smaller number of false negatives which makes a good screening test. That way fewer true positives are missed. The Western Blot is more specific meaning a smaller number of false positives. A specific test misses few true negatives. It is done for confirmation. You can always repeat the tests in case of specimen mix-up (which I have seen happen before.