I keep my AC set at 75 or above and ive had no problems keeping the ambient temps in my tanks in the mid 80's with a warmer side near 90. Maybe try covering the tops of your tanks if they are open screen. Adding a humidity box on the warm side will bring up the temp a couple degrees as well as keeping the humidity at a good level. If you are not using a good under tank heater controlled by a thermometer herpstat of some kind, and some form of overhead heat, it will be very difficult to keep the temperature gradient at the right levels if they are in regular open top aquarium type tanks.

You can also try a ceramic heat emitter. The only problem with these is they get very hot depending on wattage and can only go into a metel lamp with a ceramic socket rated for high temps. The type you can only find at hardware stores (metal drop-lights) work best for these heat emmiting "bulbs". But these should always be plugged into a thermostat/temp control unit with the thermostat lowered into the tank near the bottom at the level the animal is at.

This is what i had to do while im waiting for my enclosed caging system to arrive from boaphile.