Sounds like a flame detector problem. Look in the manual to figure out where the flame detector (item 106, page 35) is inside the unit. Turn off the power and gas before diving in. Take a ScotchBrite or similar soft plastic scouring pad and buff up the business side of the detector,see if that keeps it from recurring.
Flame detectors are susceptible to chemical contaminants in the combustion air- if it's outside an open kitchen window pr range vent where you're putting a lot of suspended fry-o-latorgrease in the air or something you can bet on it having an issue sooner or later.
Word is Takagi has a pretty good help line- they'll talk you through almost any diagnostic issue.