Single over cooker turns itself off after 40...

Question

Hi,

My cooker is about 18 months old and I am not the original purchaser so have no information or instructions about it. My problem is when I switch it on it cooks perfectly for about 40 minutes and then switches itself off. If I go back an hour or so later it will then work and after an hour or so switches off again. I have tried switching off at the mains in case it is something to do with the timer but this has not helped.

Canyou help me I amhaving anightmare cooking Sunday Lunch! and dread Christmas.