hmmm.....I don't use dimmers due to the fact that you are always checking to make sure it is ok. I would suggest getting thermostats....I know reptile basics has some reliable ones that are fairly inexpensive. Also when you are using your temp gun...make sure you are taking the temp on the substrate as doing it right on the glass will give you wrong readings. It sounds like you are having a lot of fluctuation. To keep up your ambient temps you could foil off the top screen part of the tank. Also make sure it is not near a window or a air conditioning/heating vent. We keep our house at 78 during the winter and the summer due to our animals. Try to shoot for 78-82 cool side and 90-92 warm side. Sorry I don't have more info for you.