I have a TAKAGI FLASH T-K2 tankless hot water heater. It is 9 years old.

Intermittently: the water won't heat, or the water will start to heat and then stop heating, or it takes an escessively long time to heat. This happens at the faucet furthest from the water heater, which is a kitchen mixer valve faucet. The faucet worked fine for the first 9 years. Rarely, it has happen at other faucets - which have a dedicated hot valve.

There is not a numeric error code panel on this unit; however, when the water is not heating the RED LED light is FLASHING and the GREEN LED light is OFF.

The troubleshooting manual for this issue indicates this could be the: Igniter, Flame Detector, Main Gas Valve or Pre Post Check.
When I unplug the unit and plug it back in at this time, the flame fires and everything works.
This indicates to me that the Main Gas Valve is not the problem.

Thus far I have cleaned the input water filter and flushed the system with vinegar, which turned the vinegar a light blue. I have also tested and there is not a plumbing cross flow.

The toggles are set as follows: (for 122)
113 on
104 on
NG on
LP off

OUT off
blank off
IN off
176 off

Step by step instructions are always a plus. How do I proceed to fix this problem?

Please Advise, Thanks.