AEG Competence B2100-1 oven

Hello, and many thanks in advance for all help, the above oven shuts down 30/40mins after reaching set temp along with the 2 oven lights, have fitted new Thermostat and new Rear Fan Element still prob, clock is correctly set, circuit breakers are all good,oven was new installed new home now 9/10yrs old reg cleaned and good order.On a pension, from past work able to carry out above work, thinking either of the above was the cause but no.
Again many thanks for help.
Regards verconica45