UK Car Repair: Vauxhall Astra 1.6 automatic, vauxhal astra, vauxhall astra

Question
My vauxhal astra has been cutting out on/off for the past 18 months.  For the last 3 weeks, it's spent more time in at the garage than it has with me.

The emmission control light is on cnstantly.  The EGR valve has been changd twice.  The HT leads have been changed and so have the spark plugs.

The problem is especially worrying as it sems to happen more if I use the 'S' button, which I do when excelerating on dual carriage ways or on the motor way.

Any suggestions?  The garage keeps telling me there's no problem, so how come the emmission control light is on and 'spection' comes up hen ignition is turned.  

Answer
Has the mechanic ran a diagnostic scan? This is essential to give a pointer to the origin of the problem.  If they don't have the equipment, you need to find someone who does.  As a caveat, one of the great problems in modern car repair is intermtittent loss of power, often it is so fleeting that it does not log as a fault and of course, when the mechanic gets it, the fault is not there!