Peugeot 406 cutting out and display reverts to year 2000?

I have a 2002 Peugeot 406 estate. Recently its started to cut out while approaching junctions but starts straight away although there is momentary panic as the steering and brakes lose their effectiveness. Dash display flashes up anti-theft with a key symbol. There is no alarm fitted and I wasn't aware our car had an anti-theft device anyway. Also, the calendar reverts to Jan 1 2000 although it does not reset any fuel consumption data or lose the built in sat nav destinations we added. Does not just happen when cold and is intermittent. Did it 3 times in a row just now, stopped at home for 30 mins and took it out for a test...needless to say no problem.