Well i would first start with the heater for your tank; if it is turning on when the temp is well above the set temp, then the thermometer of your heater may be outta whack a little or all together broken. Try turning the temp setting down and it may work out fine that you just need to set the temp 2 or 3 degrees lower than your desired temp. If it is broken and is staying on all the time then I would just invest in a new heater as it is nto worth saving 20 bucks from buying a new heater to find out one afternoon that your temp is 85 degrees + and your whole tank is in stress with a possibility of death for everything. If your tank is still staying too hot then try raising your lights off the surface of the water a few inches. This will allow somemore circulation of air above the tank and some more heat to dissipate into the surroundings instead of going right into the tank.
On the temp issue too, 78 to 80 is around good. 74-76 is on the colder side while 82+ is too hot and really just not necessary.