You could try using a heat pad on one end of the terrarium and removing one of the heat lamps. I always find heat lamps to be great for drying out terrariums but not necessarily for heating :P. One thing you could try, and this is how I keep my 12 year old ABF, is to put a large shallow water bowl in the terrarium and let him sit in that all the time and feed him in there. Works well for me. You could even put a heat bulb over one end of it. Lastly, I personally don't think it's necessary to keep ABFs that hot, certainly not mid 80s.