how can i fix my faulty grill, hotpoint BD51?

My grill heats up fine to start with then after approximately 15 minutes or so, usually when something is part cooked a distinct "click" can be heard and the grill stops working, the elements cool even though everything is still switched on. once the grill has been in use i can hear a buzzing noise which i think is something vibrating and i'm not sure if is connected, more annoying than anything. once the grill has cooled for a couple of hours you can turn it on again and it will work fine, but then after 15 minutes it "clicks" and goes off again. i have rang a local repairer for advice, thinking there may be a thermal cut out, but he says not. i cant afford to pay someone to come out to it but if i know what the fault is i'm happy to repair it. any help or advice would be appreciated. sean