I would guess that the heating element in the oven has a fault - as it warms up and things begin to expand with the heat someting that was nearly touching - touches and causes a short to earth - your RCD trips out turning everything off as a safety measure.
There's far too much for you to be doing than to mess around trying to mend the oven - call in an appliance engineer.
Good luck...
Peccavi, October 2009