I had the same problem using the Water Heater Thermostat. It allowed wide swings in temperature before turning on or off the light bulb, in my case. I have never tried another method of mounting it to avoid that problem but it would be worth a try. I have continued to use the Lamp Dimmer. It does require ones attention but you don't have a feeling of false security. If kept in a stable place, it works great. I use the homemade one for a hatcher only and it is only for three days so it work well.