Normal temperature control of the cooker is achieved via the thermostat and there will almost certainly be a thermal cut-out which operates at a higher temperature – to protect the cooker in the event of a fault.
Based on you post, it is likely that this thermal cut-out is operating at a lower temperature than it should. It may be that the cut-out is faulty, or it could be that it has moved out of position, and is now too close to a heating element within the cooker.