its a vented dryer so has to be an airflow problem
this dryer works by pushing the air out of the dryer. this forms a vacuum in the tub and sucks the air in over the element
so you either have.
a blockage in the airflow out
blocked filters ( which i know you have checked )
faulty drum seals. if these are faulty the air gets sucked in instead of going over the element
so you looking for anyway the air can get in circumventing the element
Dave