Neff U1721 not maintaining temperature?

Element replaced last summer and recently temperature is not maintained when food is added. Starting oven from cold and empty seems to be reach temperature (checked with separate oven temperature gauge, and red light switched off), but as soon as you add food to be cooked the temperature immediatelly drops by 20/30degrees and never seems to reach the previously set temperature, typically 180/200 degrees. And if you add food to the cold oven and then switch on, it will take forever to reach higher temperature, infact it will never reach it, the maximum seems to be at about 150 degrees. This oven is about 9 years old and that period we have replaced the fan oven element 4 times. Maybe its just getting old and tired and neeeds to be retired. Please can you advise what course of action I can take Regards Gerald