Takagi T4 starts with cold water use

My new Takagi starts running and heating water when I turn off the cold water, like the garden hose. I installed a backflow preventer gravity valve then a spring type valve on the cold side feeding into the Takagi. Both did nothing. If I turn off the cold water slowly...there is no problem. The water pressure is 85 psi. How can I prevent it from coming on uselessly?

I theorize there is an expansion tank somewhere absorbing the water hammer and the tankless is sensing the water movement.

There is no expansion tank unless there is one inside the Takagi. I was thinking of installing one to reduce the abruptness when the water is shut off. Also I was thinking I could reduce the pressure to 50 lbs with a pressure valve, or is 85 psi OK?