I wonder if your vent is clogged, or sometimes the coils are not enough to correct the problem, and the gas valve has to be replaced. So we need to isolate the problem. I realize you have a gas line, in the back, but pull the dryer away from the wall as far as you can, without damaging the gas line, and take the vent hose off of the dryer at the wall, leaving the end on the dryer. Lay the vent hose in a way, that it will blow the air out, without the wall obstructing the air flow, and turn it on. If the flame last alot longer, you have a clogged vent. If not you need a valve. I would imagine, that you have taken the front off, this dryer to be sure that the blower housing is not full of lint, if not check it too. Tom ApplianceEducator.com