Hi. Well I had/have a similar problem, and it turned out to be the pumps producing heat! I had three pumps running (chiller, skimmer, main) and the chiller was constantly trying to keep the temp down, and was unable to keep the tank below 83°. This was with LED
lighting which is virtually heat free. But it turns out that 82° or 83° is not terrible depending on what you keep in your tank. I have mushrooms, a fungia, a closed brain and a butterfly, chromis and two dwarf angels, and they thrive. My temperature is high, yet consistent. An important thing to realize with a chiller is that you need a place to send the heat that it produces. Just like your refrigerator at home, the heat that is removed from the tank is displaced elsewhere, and added to (think increase in entropy from physics class). So many people place their chillers in another room. If your chiller is very close to your tank (i.e. in the same room) you will heat up the room and the chiller will have to work harder, releasing more heat into the room, heating the water, making the chiller have to work again, etc.
Chillers are great but they do have limitations, requirements in setting up.