Gas fire will not stay on. Ignites for 7-13 seconds and then shuts off for 52+ seconds. This cycle is repeated with the Fire-On time gradually decreasing and the Fire-Off time gradually increasing for each subsequent cycle. The Thermostat Fuse has Continuity. The Igniter resistance is 61.7 ohms. The Flame Sensor has continuity. The Gas Valve Solenoids have been replaced. The vents and ducts are clean and air flows freely. What are the test procedures for the High Limit Thermostat and other remaining components that would cause this anomaly? Any other suggestions? Please help!! Thank you.