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Smith GV100w shuts off

I have a smith gv100w boiler with fenwal controller. At TFI: the igniters glow yellow for a minute, but there is no gas and I get the 3-flash indicator of ignition failure. If I turn off the power, wait a few minutes, and try again, then it will sometimes start OK, and the burners stay on long enough to heat up everything, but once the reaches the target temperature and the burners stop, it won't relight on its own - I have to do the manual power off & wait a few minutes. My technician replaced the fan pressure switch, but it didnt solve the problem. During the time that the igniters are glowing, I can see 24v at MV1. I am waiting for it to stop working again to look at the voltage at the gas valve but it is at 24 volts AC when the machine is working. I hear the ignition clicking noise if the burner switches on and I hear it more loudly when it doesn't switch on-- it makes a loud clicking noise and then the red lights start blinking. What should I look at next?