My 3.0 had a similar fault when it was about a year old and it was caused by a faulty sensor. Sorry can't remember which one but it is easy enough for the dealer to read details from the computer. Regarding other comments about lack of power from the engine, mine is the later 210 hp version with auto and once the revs get beyond 3000 rpm it really flies. Remember though that the 406 is not a lightweight car and it will never accelerate like a hot hatch. Also that the engine is so smooth and quiet there is little sensation of power and you probably don't notice how well it moves.
Also (I suspect the same fault ) it will not run at all, the engine dies and has no power but the engine ticks over. Wait a short while and off we go again.
Might this be a duff sensor, if so where is it.
Many thanks and best regards Julian Wesson