my idle is still not stable. The valve was rather gunged up and I really thought that this was the problem. However I cleaned it until it is perfectly free only to find that my idle speed is still not completely stable all of the time. The fault seems to be an intermittent one because sometimes things are fine and the engine idles sweetly. Then when I stop at the lights again I feel a slight shudder as if one or more cylinders are mis-firing. Why would it come and go like this ? Someone suggested changing the spark plug insulating boots ? I have cleaned them and they look fine as do the plugs themselves but I wonder if new ones would cure the problem, can the rubbers be bought seperate from the coils ? Maybe I should I be paying for a computer diagnosis ? Any suggestions please ?