There's a sensor on the thermostat housing with a 2-wire plug. That's the fan switch. Unplug it & use a paperclip to jumper the 2 wires together. Key on, that should make the fans run. If this works, then the fan switch is probably bad, get a new one. If you're prepared to swap quickly it probably won't dump too much coolant out of the hole.

If jumpering that plug sometimes works & sometimes doesn't, then we'll talk about checking the fan relay, or looking for flaky wiring connections.

If the temperature stays good while you're driving, then the thermostat is OK.