This type of problem is typically caused by low water flow.  Noritz tankless water heaters have a minimum flow requirement (0.5 gpm for most models.)  If this flow requirement is not met, the heater will not fire.

If you heater shuts off during usage, here are a few things to check:

• Ensure your filter is clean and free of debris (instructions)
• Verify gas, water, and power are supplied to the heater
  
• Try lower your water temperature to 120°F.  This will ensure you are using more hot water and meeting the minimum flow rate requirement.