I do not know how old this unit is or what model but many water heaters are not sophisticated enough to measure the temperature, but heat based on water flow and temperature rise. To start, tankless water heaters turn off every time you turn the hot water off. Lets use an example. You use the facilities, wash your hands then turned off the water and take a shower. You will bring the already heated water (from washing your hands) from the pipe up to the shower. Then the cold water that comes into the pipe from the reactivation of the water heater comes up the hot water pipe with warm then hot water just behind.
If this is not your problem, keep reading. Every unit I have been called to see were installed wrong. Check your installation manual to see if the problem with the system started with the contractor.
First, check the temperature of the water from the heater. If it is too hot and you are mixing too much cold water in, the unit will turn off because there is not enough water flow to keep it running.
Second, turn off the valve at the water heater and turn on the hot water at a faucet. If the water does not stop running, there is a "plumbing cross over" usually caused by a failing pressure balance valve in a shower.
Third, if the unit has not been serviced, go to bosch-climate.us to find a tankless water heating professional in your area. Totally Tankless LLC