Yhis heater requires a minimum flow of .75 gpm or more. If the flow drops below .65 gpm, the burner will go off, making the wwater cool. The instructions tell the installer to purge the system of dirt before installing the unit.
Even so, if there has been some dirt further up the line that could come through, the tubing may have become partially blocked reducing the flow to below minimum. Have this checked out. There are some reasonably priced flow indicators that would show problems in the future. A low restriction dirt-catching filter could help eliminate further problems.
European and Japanese users are used to having the repair company come to disassemble, check, and flush the units on a yearly basis. They do not expect the unit heat exchangers to last more than 5 years, so they trade longevity for energy savings.
Check the web site at: http://www.galacticsalvage.com/takagi_techsup.html#no