weird AO Smith GCV40 gas burner cutoff issue

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Aug 1, 2020
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I have two 2006 AO Smith GCV40 tanks. One for the first floor, one for the second. They share an exhaust vent. The vent passes the draft test. The units are in an elevated closet in the garage and not exposed to lint or dirt.
I take very good care of them, I change the anode rods as needed, test the valves and flush 1-2 times per year. I know they won't last forever, but they should last a lot longer than the the ones in installed in most homes in the area.

I woke up one morning to find the pilot out on the unit that services downstairs. It wouldn't relight. I took the burner assembly out and the thermal fuse had no continuity. So I ordered a new assembly which is the igniter, thermcouple, pilot light and door insulation. I also completely cleaned out the cordite disc top and bottom as well as underneath the heaters (I did both while I was at it) as I am aware of the issues with these heaters collecting dust/dirt on the disc starving the heater for air. The inside of the chamber and the burner were in great condition. I also checked all connections... no issues that I can find. It started right up. Since the tank was cold, it took a bit of time to get the temperature back. The flame color and appearance was normal, no ghosting or weird flame movements or coloring. It was the same as the other identical heater. I could even hear the air moving up the venting and it again passed the draft test.

Then, the burner and pilot cut off about 8 minutes into the first burn. I witnessed it once, and it was a quick shutoff, not a gradual thing associated with lack of air or gas. From what I understand, it seems like the TCO (thermal cut off) was tripping. It does this now from time to time, typically only when someone takes a shower which suggests the burner is on for a while, and it's not consistent, it can go days without an issue.

I am out of ideas. It's either the gas control valve (although I don't think these go out like this?) or some really weird venting problem that doesn't impact the other heater that shares the same vent ductwork or a faulty thermcouple setup (which seems odd, it's brand new and the pilot stays lit). I just yesterday tried to swap the whole assembly under the tank between the heaters and will see if the problem follows the tank or the assembly or if it goes away completely (which itself would be odd).

The gas control valve is $130, which is on the border of spending too much for such a heater I think.

I also have CO detectors nearby and there's no evidence of back drafting.
Any ideas or thoughts? It's really strange, as I have addressed, I think, all of the normal problems with these heaters already and unlike most people (at least in my neighborhood) I do regular maintenance on them and they are clean.