It may yet be overheating. I live in a similarly warm environment (Houston, TX) and also have mine mounted outside in a sealed utility box. I added a laptop exhaust fan to the box and a simple controller for active cooling, but for the first two days after installation I ran it with the fan shut off just to see whether it was really necessary. On the second day my Raspberry Pi stopped responding to ssh, RDP and ping, requiring a reboot. I wasn’t logging temps at the time but I assume it was due to the 85°C CPU shutdown limit. After that I configured it so the fan starts any time the CPU temp exceeds 50°C and it has been trouble-free ever since (even this weekend when it was 104°F here).
If you are operating via WiFi and don’t need wired Ethernet, I would suggest that you use a Model A rather than a Model B. The A runs 15-20C cooler than the B under similar load… the LAN controller chip on the B generates a lot of heat, even when it’s not in use.