My 2.1TD 1998 406 starts instantly and runs smoothly when it's dry and even when it's freezing (new glow plugs), but after driving a few minutes and the engine not quite hot yet it starts to idle rough. i.e. when stop at a set of light half a mile away. When it's wet it idles slightly rough straight away. When the enigine is nice and hot, after 20 minutes of driving etc the idle is nice and smooth. Any ideas?