I had a similar problem where the AEG Competence D8100-1 oven would switch itself off, whether it was oven, defrost, grill or any setting. You could switch it on again, but while cooking a single meal it would do this 4 or 5 times, getting worse over the years.
I discovered the actual ON/OFF switch was probably the cause, as even the gentlist of touches (even a slight swist without any pressing motion) would duplicate the fault.
Idealy, I would like to have replaced the switch but I settle for cleaning it with electrical switch cleaner this time.
It's not easy to get to though.
First, remove the mains supply to the oven.
There are 2 front screws holding the switch in place, but these are hidden (in may case) behind the white glass control panel cover at the top of the oven.
This glass panel is factory glued in place, but with a blunt knife I managed to remove the glue from one side and the bottom of the glass panel. This allowed me to swivel the glass panel upwards and access the 2 screws holding the switch in place.
Next remove the top access metal panel to the oven. On top of the oven there are 4 screws and a couple down each side. This will involve bringing the oven forward a few inches, watch it doesn't topple over.
Once the top metal panel is removed you will see the main ON/OFF switch. Make a drawing of all the wires and connections (so you know where all the wires go back when you refit the switch) and then remove all the wires from the switch (about 10 of them). You can now clean the switch with electrical switch cleaner.
If you feel realy brave you may attempt to disassemble the switch as it appeared the plastic cover was only held on with 3 plastic clips. However, the switch is not designed to be repaired (but I have done many in the past) and removing the cover may involve spring, contacts and levers flying everywhere and you may never get it back together again.
Once you have cleaned the switch, reassembly is the reversal of the above, taking care the switch wires are connected exactly as they were before.
If you can source a replacement switch on the internet or anywhere else, now would be a good time to replace it.
Hope this helps someone.
Dave M, October 2009