Heater Help Please

I have a 55 gallon freshwater tank just fish two hang on filters and pump with airstones. Just a couple of days ago I noticed my heater was on more I kept an eye on it. The temp was set at 76f i noticed the temp had risen to 80 then to 82 the heater light still on so I turned it down to 73F on the heater. kept an eye on it it appeared to stay 76-78f for two days. woke up this morning and temp was 82 heater still running. I bought a new heater today set it to 76 watched it and the same result heater stays on even when water is 78. I have 2 thermometers one glass and one digital at opposite ends of the tank so I know my readings are correct. Any suggestions do I have two bad heaters this is very stupid I just cant figure out why. Please Help

they are fully sumersible I used the suction cups and hung them verticle so the bottom of the heater was about an inch or inch and a half from the bottom of the tank

I have pretty good circulation I have had this tank set up about 6 months with the heater in the same spot and have had no problems in the past with this.

its a hydor Theo heater 200 watt it has a it has a temp setting with deg on the selector i set it in the tank unplugged for 20 min set it to 76 and then plugged it in I will try turning it down to see if that works.

That is what i wanted to hear I feel better now that someone has the same problem. I will experiment with that good thing I am not seeking 73 or 74 I would not even be able to turn it down anymore crapy heaters anyway.