Dryer Shuts Off Before Clothes Dry

I have a Kenmore dryer that has "auto dry". I’m assuming that means it looks at the humidity level in the air and adjusts the run time based on that. I cleaned out the "grill outlet screen door", lent screen cover yesterday becuase it was taking several runs to dry the clothes. I tested the dryer, it got hot and dried clothes. A few loads later, it would not run on automatic anymore, it would shut off about ten minutes into the load. I can run the dryer on a "timed dry" without problem, but would like to know what the problem may be. No owners manual, so any help would be greatly appreciated.