Hello; Might be time to consider using two heaters. I like to do this in a tank as large as you mentioned. This way if one fails the chance of the tank becoming too cold or too warm is much reduced. This practice can, and has, kept tank temps stable for long peroids. I sometimes have, as now, lived 50 or more miles from a decent fish shop. I can have time to plan a trip or mail order.
It also can make the temp more uniform in a tank. I have also seen fish gathered around a heater, sometimes due to having not enough power for the tank and other times due to a lack of flow during a filtration issue. I also "suspect" that I have observed fish staying away from the area near a strong heater.