I actually have a very similar (same?) problem with a 50 gallon gas water heater manufactured by State. I’m in Texas and the water heater is in the attic.
Only during very hot summer days (100+ temp) the pilot goes off by itself. It does not go off unless attic temperature is very high so I only have the problem in summer. Other months no problem at all. I changed the thermocouple twice but the problem continues. The vent goes straight out to the roof and it has a vent draft installed on top of the tank. The vent pipe sticks out of the roof about 2 ft. A handyman friend came by and he thinks the vent cap out in the roof is too small. This seems to agree with Larry’s comment about low air through-put.
Is there a recommended vent cap size for a 3.5″ vent? the larger the better?