Rather than troubleshoot the cause, I replaced the Duct heater assembly, thermistor, and thermal fuse. Also replaced the dryer belt. The dryer belt was very difficult to remove from the motor pulley. It took many tries before I was able to remove the belt. The belt can only be accessed from the vent pipe opening in the rear of the dryer. And just one arm can work for the belt pulley assembly.
I couldn't replace the belt on the motor pulley so, after numerous tries, I removed the drum and tied a rope on the tension bar/spring. After replacing the drum / belt, I had my wife pull up on the rope so I could then easily put the belt over the pulley. Then untied the rope and removed it. This worked very well. Replacing all parts was easy except for dryer belt.
Also cleaned out the lint buildup in the blower and all parts inside the dryer.
All parts ordered and received were exact replacements for Samsung Dryer.
Dryer now seems to working properly, and seems to run much smoother with all of the lint cleaned out.