I think Mike is on to something. You either have a probe problem, or a controller problem, or a heater problem . Something is telling the heater to shut down with an actual internal temp that is less than the set temp. Or, all the heat it can put out is not enough. For my AQ, I've never tried to cook as high as 300, so I can't say what my cooker's response to that is. It might be that that's the best it can do if your doors don't seal well and insulation is old. Best call CS.