Pentair Minimax 250 Millivolt heater problem

[nrockow]

I have a 6 year old Pentair Minimax 250 Millivolt heater. The problem that I am having is that when the heater runs for a long time and the whole unit itself gets hot (just starting up the pool so it is taking some time to heat up) the controls stop working. I cannot turn the heater off with the on/off switch or the thermostat control. I have to turn the gas supply off. Additionally, when I run the heater with the panels off every thing works fine, although I have not run it for a day with the panels off, I have run it for the same amount of time off as it takes for the controls to stop working with them on. Another symptom is that a couple of times after I turned off the gas supply and went to re-light the pilot I didn't even have to hold the pilot knob down. The gas was coming out as soon as I turned it to pilot. Not sure if that is supposed to happen or not. Also, when the heater is running and the controls will not turn it off, breaking the loop of safety switches will turn the heater off. Thanks in advance for the help.

[nrockow]

To add to my previous post: The last time the heater got hot enough that the controls would not work I simply disconnected the safety loop at the gas valve to stop the burner. The switch was in the off position (if the switch is not in the off position when the loop is disconnected the burner will re-ignite and the controls will not work). About an hour and a half later I turned on the with and the heater fired up and the thermostat and the on/off switch work fine.