I would more than likely conduct a missed approach, however using my knowledge from an article i read in flight safety, it is possible for the PAPI to show an errounus reading due to refraction from particles in the air making an "on slope" light indicate fly up or fly down. The way in which to distinguish if this is happening is to see the intensity of the lights, the artile states that the erronus light may be dimmer then the correct lights.
John