Hello: Outside air temp does play a part when the heater is located in an attic. That’s something I’ve heard of and dealt with.
My point about thermocouples and controls is that there is something in them, in your heater that’s weak and temperature is affecting it. A touch of corrosion at the thermocouple/control junction could be your problem, but thermocouples are cheap and replacing it eliminates that and other possible problems all at once.
It’s never been an option for me to leave a client’s house before their hot water problem is fixed. Experience is that between thermocouple, gas control and looking around for other obvious problems, (nest in the vent, rain coming down the vent pipe, no gas, big hot water leak, gas control set wrong…) I’ve always been able to take care of my clients. I’m very interested to know why something misbehaves, but am sharing with you the how of getting to reliable hot water in the quickest fashion.