Dryer often runs controlled by high limit thermostat instead of the air output thermostat. This prevents the dryer from stopping on the auto timer modes where the timer only advances when the air output thermostat opens. The vent pipe is fairly long (about 17 feet) which may be part of the problem. If the vent is absolutely clean, it will run on the right thermostat, but it seems that any lint at all in the vent will cause it to run on the high limit stat. Would it make sense to reduce the gas flow about 20% so that the high limit stat doesn't trip too soon? With full gas flow there is a lot of flame near the high limit stat. With the gas turned down a little, the temp in the burner should be lower, so the air output stat should get tripped before the high limit.

Does this make sense?

Thanks.