Dryer won't dry...intermittent flames

When turning on the dryer (Either Automatic Dry or Timed Drying) I can see the ignitor (I'm guessing that's what it is) glow for a few seconds, then go away. Then about 20 secs later or so, I hear a loud click (ignitor I believe) but no flames. Gas valve is open. Then I'll unplug the power, turn off gas, turn gas back on, plug in dryer, turn on again, and after the ignitor clicks, Whala!! the flames are there...but it only lasts for a few minutes. Gas line is not kinked, vent is clear, exhaust clear. Any ideas?