Hello; I guess the old style heaters which use contact points are no longer made. I have some from decades ago. Every few years to a decade or so the contact points can deposit a small tit which keeps the heater on or hard to adjust. I do take them apart and file the points.
Do as duanes suggests. Have several lower wattage heaters in place of a big watt one. I like to set a lower wattage heater to come on first and a second or third higher wattage to come on a degree or two higher. The low watt heater will stay on and the others only on when the low watt cannot keep up.