Unless you pay big bucks for an incubator, you will have fluctuations. Be sure and use a proportional thermostat, that will help. A regular thermostat or dimmer, turns on, and off. The proportional thermostats will pulse small amounts of electricity to keep the temps steady. Also try, and keep room temps steady. A 2-3 temp variation is not bad. Just be sure in your case, that the higher reading stays at 90, and not any higher.