Cleaning the lint filter after every load is very good, however you probably have a blockage in the vent system. The dryer manufacturers recommend in the owners manual cleaning the dryer vent system every one to two years.
The dryer vent system includes the transition hose (slinky tube) from the back of the dryer, the ducting running to the outside of the home and the vent louver where the exhaust exits the home.
It sounds as if your system is completely or almost completely blocked.
If your exhaust exits through the roof it may be best to hire a professional company to clean the vent.
Also with the amount of blockage you must have it would be very advisable to clean the dryers internal ductwork as well. This includes before and after the exhaust fan and the ducting to the exhaust port where the transition hose connects.