I have been using Jager and never had any issues with them so far. Currently I have 2 300 W in 180 gal and the water temp stays quite accurate. I especially like the fact that their surface area is relatively bigger, allowing better heat transfer rate.
From what i know as the heaters age they are more prone to causing issues. Some hobbiest with very expensive fish prefer changing them at certain time intervals. The important thing is that the heaters don't actually adjust themselves for the water temp, rather they are adjusted for their internal temp. Depending on the temp difference between the water and the heater, heat transfer rate differs. Another component that will greatly impact the rate is the convection rate. So if you set your heater at the far corner of your acuarium where there is no current for the desired water temperture and move it close to the current later on, that can cause higher temp. all of a sudden.
Another contributer is the ambient temperature and other heat sources, such as direct sun light. If the malfunction occured during the night time, I am assuming it wasn't any hotter than normal temp in your place. If your temp dropped down when the house is a lot colder i would assume the problem be the ambient temperature, but while the outside temp is steady or going down, your water temp shouldn't go up.
Readjusting the heater might work for you so i f i were you i would give it a try. But if you dont want to risk your fishies readjust until you get a new heater, my preferance Jager.
