This is a common problem with projects using Arduino's boards based on esp8266 chips. It's called a "watchdog reset error" and is caused by something in your code, or one of the libraries used, which is blocking the chip from managing the WiFi connection.
It sounds like this error started appearing when you added the code for the thermocouple, so it's probably related to that. Perhaps the library is not esp8266 compatible, for example.
How are you connecting the max6675 to the Wemos? Please post a schematic. Pen and paper is fine.