The ignitor is either going to be good or bad. If it fires off it is fine and if it is shutting down then the flame sensor is also working. That tells me your coils are weak and old.
Here is how it works and why I believe you need to get new coils: When the dryer is cool first thing in the morning the initial start up the fuse and control beside
it allow power to the gas assembly, the flame detector/sensor allows power to get to the ignitor and it begins to glow. 1800 degrees is what that baby puts out and at that point the flame sensor bi-metal bends and shuts down the ignitor. The holding coil and secondary coil will then have enough power partially because the ignitor is off and partially because the ignitor is now acting as a thermocouple. If for some reason the gas did not ignite then the ignitor would cool down and shut off the gas.
When the temp inside the drum gets to 160f that control beside the fuse shuts off the gas assembly, IF it don't shut down the gas then that fuse melts and shuts down the power to the gas assembly but it doe not shut down the dryer. The fuse is good though because you are getting power to the gas assembly AND the gas assembly is shutting down at 160f. The dryer keeps a tumbling around till the temp in the drum drops to 105f at that time the dryer wants another blast of hot air, trouble is the coils have gotten hot now, the ignitor is doing its thang, the flame sensor is doing its thang, the control is doing its thang. The only thing that ain't happening now is the coils ain't allowing gas to come out. Replace the coils and party on! More here
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