It's almost definitely a lint buildup causing poor airflow. I am actually surprised that a thermal fuse didn't break. Even if the ducts are cleaned, the fan blades will hold onto a fair amount of lint. I would suggest checking there first. Most dryers I have disassembled have a cover plate over the fan assembly in the rear of the appliance, but some require a little more disassembly to get to the fan. If you can give us an exact model number, we might be able to help diagnose further.