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Your model has been discontinued for quite some time so there are no specs available on the controls. If this is electronic ignition what you describe typically indicates either a dirty flame sensor, a bad flame sensor or a faulty ignition control. Could also be a bad connection. In these models the pilot electrode also acts as the flame sensor. If the electrode continues to arc after a pilot flame is established then that's a definite indication of a flame sensing issue. You can try carefully cleaning off the electrode with fine aluminum oxide sandpaper as a first step.