I have a Delonghi Range cooker. Model # DSC 95 DF FSD. The oven and light does not come on straight away like it used to. I have had this cooker for about 4 to 5 years. In the last four or five months the oven takes longer to start heating up - between 20 - 40 mins. When the light comes on then it starts to work as it should but that is usually about 30 mins wait. Before everything worked straight away. Is this a common problem? Is the thermostat on its way out?