In the heat of an El Cajon summer, the water coming into the house is already warm, so a tankless doesn't have to work very hard. When the first cold mornings arrive, everyone tends to shower at the same time. A unit that had an easy summer suddenly has to work much harder, and a problem that's been building is more likely to show up.
Hard water makes it worse. Scale builds inside the heat exchanger over the years and restricts flow. The unit can run hotter than it should, and eventually it may shut down to protect itself.
We'll get someone out to find out which it is, a descaling, a part or a unit that's done, and you'll hear it in plain English. If it's time to replace, the new one gets sized to how much hot water the house uses at once.