Tell him to check the cooling fan.. If the cooling fan is not working the thermal overload will cut-out the oven to protect you. If the fan is not working, check its thermal switch.
Before you change it... Is the model that you have, one with a clock/timer?
So, when the oven switches off, does the clock go out ... If yes, when it comes back on, is the time back to 0:00 or has it remembered the time?
So it goes from Manual to Auto mode, forcing you to re-select Manual, yes?
Then the issue is with the clock/timer and its relay...
Depending on the timer you have, it is possible to bypass the timer.. If your timer has spade connections for its wiring at the relays then it is possible...
Get the electrician to take a photo for you and I can see if I can instruct you how to do it..
You are very welcome