A UV sterilizer should be plenty safe to run on an outdoor pond with koi, considering it is hooked up properly. Besides ammonia, nitrite, nitrate, and pH readings, can you also post KH and temp? It sounds as if shock of some sort could be the issue, but without knowing any specifics about your situation, it's very difficult to do much to help you.
How big is your pond? How many fish? How big are the fish? Was there a specific reason you added a UV to the pond? Also, what part of the world/country are you from? Weather changes can also play a big part in pond keeping.