If your dryer is clean of lint and your vent is clean as well then the dryer will still only burn flames for about 30 seconds. This is the norm. The dryer constantly cycles during the drying period. What about your blower wheel? If it is stripped the dryer will take twice as long to dry clothes because the air flow aint right. Have you checked it to see if it is stripped? This is a common problem with these dryers.