The first possible reason is a build-up of grease on the grease/drip tray that is catching fire and causing the cooking chamber temperature to exceed the set temperature and the pellet grill to shut down.

The second possible reason is you either have a dirty or faulty RTD probe (the temperature probe inside the cooking chamber). First off, with a cloth gently clean the probe to make sure its not covered in grease/fat, and then test the pellet grill again to see if it can maintain a more consistent temperature and not overheat.

The thing with RTD probes is they are very fragile, if they get knocked (especially while hot) the thin wire inside them can snap. Therefore, RTD probes do need to be changed from time to time.

If swapping out the RTD probe does not fix the issue then that may mean that there is an issue with the controller. However, before you spend the money swapping that out, if your controller does have a P-Setting I would encourage you to spend a bit of time playing with that to try and address the issue.