From the sounds of it, you're not taking the surface temperatures of your tank which is what you need to be concerned with. You can achieve this by using a temp gun. I say this because I don't think your house temperatures are going to influence huge 10 degree swings in surface temps; especially if you have a working thermostat controlling your heat mats. It sounds like you may be using a regular thermometer to take air temps inside your tank. Does this make sense?