I had this same problem back in 2008 and through a long troubleshooting process determined that the burner "sustainer coil" was failing. The coil would "go open" after it would heat up. Installing a new set of burner coils resolved the issue. While performing this repair I installed a new felt seal on the back of the drum and replaced the drum support rollers. Basically, a $190.00 rebuild saved a total replacement of the dryer.