Plenty of Santee households hit the hot water hard first thing in the morning, with showers, the dishwasher and the laundry all going before everyone heads out. A healthy tankless keeps up. One that's been building scale for years is the one that starts cutting out.
Santee's water is hard, and the minerals settle as scale in the heat exchanger. As it builds, the unit has a harder time keeping up and starts protecting itself by shutting down. Sometimes the cause is a worn ignition part instead.
Someone comes out, finds which it is, and gives you a straight answer in plain English. If a descale and a part fix it, that's what you'll hear. If the unit is undersized or done, the replacement gets sized to how your household actually uses hot water.