This does not happen all the time, but after it has been running for about 20 minutes. It will shut down. You can restart it right away. I have a kenmore model series 80, and the problem as been happen off and on for about 6 months

Sounds like something is intermittenly causing the dryer to drop its latch circuit. Could be a bad spot on the timer, could be something as simple as a door switch that is loose. You will need to check for loose connections. This is most likley the issue. As the dryer tumbles, its probobly jarring something loose every once in a while.