When the dryer is started the Ignighter comes on and the gas lights and stays on for normal cycle time. Once the gas shuts down the next time the ignighter comes on it lights same as the first time however the gas does not come on. Ignighter stops glowing, then repetes ignighter cycle after a short time. Replaced flame sensor (Part 21 on gas valve parts page) , same symptoms. Once dryer cool down will light first time again. 
Any ideas?
Thanks!