What are the controls being used, SCR controlled or mercury contactor?
Voltage?
Model # of the firerod and its resistance?
how often do you have a failure?
how long has this been going on?
I don't know exactly what you are using or what kind of failures you are having. Having said that , most of my problems with flamerods were caused by poor/intermittent ground path return. To remedy that, I added an additional ground wire between the physical burner and the controller/detector.
Basically what James asked.
Diameter and length of the heater?
Voltage and wattage?
PID or On/Off control?
Are there any obvious signs of failure on the heater?
What color is the heater sheath?
Dark spots or small holes in the sheath?
Guessing this is some sort of internal failure and not leads?
Application? Is the heater inserted into a metal block or liquid?