Need help with new install of TH3-J Takagi heater for radiant floor system

I have just installed a Takagi TH3-J heater into a new construction / new install radiant floor system. Brand new everything. Closed loop system. Less than 40% antifreeze in radiant loops. Using propane. Since the 1st day the system has not functioned. It will run for a period of time (from 1 hr up to 2 days) then shut down with a 391 error. I have called Takagi service & they have no answers - only guesses as to what could be the problem. Just keep telling me how reliable the heater is and how they have never seen this issue. I had a certified service rep out & he has found nothing. He did say after his 2nd visit that the regulator on the propane tank is not responding correctly at the moment the system ignites. The pressure drops very low. The gas company did check the pressure earlier and it is well within the range specified by Takagi. They have not been back out yet to confirm the repair guy's findings. I am not sure if that is the problem - why does it run fine for a couple of days with multiple cycles of start/stop then all of a sudden stop working? If the regulator was bad - it would be for all events not just a random event. I think it is something electrical - as the system heats up a connector, harness, circuit board changes and causes a fault. After it sits for awhile & cools back down - it works normal until it gets heated up again. I ran through all of the diagnostics with the service rep on the phone with Takagi and all was good.
I am concerned that the system is so sensitive that any minor thing will cause a fault. I want a system that is dependable & durable and able to survive in the country with flies, mosquitoes, dust, snow - all of the things they have told me could cause the system to fail.

Any ideas on what could be the issue?

Thanks