Is it possible that there is a series of JBL ProTemp S heaters with in/out intervals that are not adjusted properly?
According to technical specifications, our heaters have a control accuracy of +/- 05 °C. Taking measuring errors in temperature measurement for aquarium operation into account, the switching accuracy should be approx. 1 to max. 1.5 °C. If a heater has deviations of 5-7 °C, there is usually only one possible cause.
The heater is not adjusted properly, which can be determined by checking whether the heater has or has not a push button for readjusting (JBL has several different series). Otherwise it is defective. The heater is mounted in a spot that does not have sufficient flow of water.