dryer wont stay hot or now get hot

I've taken the dryer apart to see whats going on. This is what Ive noticed. The flame does cycle on and off, repeatably, But when it lights a flame, it only stays light for a bout 30 seconds and clicks off for 2 mins then back on for another 30 seconds and so on. I've cleaned the ducts and all inside the dryer. When put back together, it seem to getting hot at first, so i tried a load, then nothing again, i dont' feel any heat again. What part do you think it is?