How to decalcify - Mountain Plumbing Mountain The "Little Gourmet" Heating Tank MT641

This could be a heating element going out do a ohms check on the elements

Open following link for Bosch tankless troubleshooting resources, including Bosch contact information, typical manuals, error codes, tech support, parts, fuel-saver calculator, service bulletins, water heater forum, fixya links, disclaimers, and links for finding service technician.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Read manual cover to cover, make sure tankless receives yearly maintenance as outline in manual.

Add a comment with your experience so others with same-similar problem can benefit.

Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

My guess would be that the heater elements are coated very heavily with calcium deposits and are not oeperating effecient enough to keep up with use. These can be removed and replaced or cleaned but the tank itself would also be suffering the same problem. I have never tried decalcifying a tank because the age of water heaters are usually more energy and money saving to just replace. sorry