Hi Phill, i have a 406 and i have had similar problems, but the other end of the scale, (over revving) i am still trying to sort it now, but i have come a long way, so start with the idle control valve, remove it clean it with carb cleaner, while its out clean the housing, remove the throttle body, and clean with carb cleaner or a brush and some thinner, then reasemble, but dont forget to disconnect the battery before you start, when you put the terminals back on, turn the ingnition on for 30 sec, (lights only) then start car BUT DO NOT TOUCH THE EXCELERATOR) leave to run until the radiator fun cuts in, when this cuts out turn off, then wait 5 mins, turn on ignition (lights only) and start still no excelerator, leave to idle for 5 mins and see how it runs. Then take it for a drive for 20 mins. Hope this helps, if not try a garage.
Lee from Bournemouth, February 2009
