Yesterday noticed the idle was down to around 600 and then would settle to normal,today while ticking over for a while it almost cut out and sounded rough,drives fine when moving.noticed the other day too kinda water gulping sound when heater switched on,,wondering if they are connected in some way,heater works fine and all else is normal.new plugs are going in tomorrow.i changed the rocker gasket nearly 2 weeks ago and all went fine there.any ideas..no leaks under car..car is a 2 litre petrol e39