A dual stage temerature controller costs around $120(new), and will eliminate this problem along with alomst any temp fluctuation at all. It also provides protection from a heater sticking on like we have all heard about too many times.
Ranco controllers are pretty common in the hobby, and about the cheapest you will find.
www.diyreef.com has them.
I use a digital aquatics reefkeeper.It is about $100 more than the ranco, but it has other features like light timers wave maker etc.
http://www.saltycritter.com/controller-index.htm