My tank runs about 77F-78F in the winter and 78F-80F in the summer and its fairly consistent, unless there is a user error which has happened multiple times.
Not absolutely necessary, but I'd get a digital thermometer that has a submersible probe.
If the ambient temperature of your house stays relatively the same, I think you should leave your lights and fans off for one night then properly set your tank temperature to about 77F-78F since that seems to be your magic number with the fans and lights running. After setting your temperature, I'd put both your FANS and LIGHTS on the same timer. I think if you do this, you'll have much more consistent temperatures.
Someone smack the pdiddy outta me if thats bad advice.