Without knowing how old the refer is we all are just guessing. If it's a few years old, mysterious shutdowns can often be traced to lack of maintenance. Cleaning the flame area, removing and replacing the connectors for the thermocouple, checking the air flow would be places to start.
As mentioned, running the refer when tilted more then 5° can cause it to stop cooling. Rule of thumb, according to the manufacturer, if it's uncomfortable to walk in your rig, then it's tilted too far for the refer.