definitely a real issue.
I have my primaries sitting on a register for the summer months. This keeps them right at about 70F. I don't know if this would work for you or not.
Other methods, involve building a fermentation chiller, either a fridge/freezer with temp control, or a "son of a fermentation chiller". Search on both of those. Also, some people keep the fermenter in tub of water with a towel wrapped around it. Since water won't fluctuate so much during the day, it can keep the fermenter cool from the cooler air the night before.