Mine has been doing this for a while. I read the forum posts and changed the part you mention - no fix.
I have today found a forum post on this link http://www.howtomendit.com/answers.php?id=257620
that says the main power switch is the problem.
I have just tested it, switched the oven on to fan oven, let all the fans rev up, and wiggled the power button, and the oven switched off exactly the same as if the fault had occurred.
I have just emailed the webmaster to see if they can supply a new switch, if not, I will have to repair the unit.