Oven knob turns up to 250 degrees on its own!?

My mother in law is complaining that her oven turns up to 250 on its own. She turns the knob to 190, puts food in and sometimes when she returns from doing chores the oven has turned itself up to 250. Sometimes when she is in the kitchen she will look at the oven and suddenly realise the knob has turned to 250. It is a Beko OIF22300XL and 15 months old so not under warranty Beko tell us. The first few times it happened we thought she had knocked the knob but these are reasonably firm to turn and now it has happened at least 20 times. We are going to sit and watch the knob to double check it is turning on its own. Has anyone any idea what we can do? (apart from sitting and watching oven for the whole cooking time - who has time to do that!)