Intermittent problems are always going to be a difficult - "fire up the parts cannon" as Rainman Ray would say.
Have you checked your coolant is clean and not increasing or decreasing in level, white smoke is generally a head gasket issue with water going through the engine. Looking at the spark plugs helps because if one is getting washed with water/steam it is usually obvious. Unlikely to be as intermittent as you describe but cars are notorious for ignoring the rule book.
There is a possibility that the problem could be caused by water in your fuel tank, does the problem happen when you are cornering of braking hard? Is the fuel cap seated well? Water comes from petrol stations sometimes. Do you normally buy fuel at the same place? Is the fault related to either recently filling up or being low on fuel? Is it related to rain or car wash? etc
Did the garage give you any indication of what fault codes are popping up? If not I have Opcom (the Vauxhall diagnostic tool) and could read them for you, there will be historical fault codes but whether they are meaningful or not is another matter.
On a non-water front, you could check all the intake hoses are well seated and no holes - air leaks on the intake assembly can cause intermittent problems including stalls, wrong mixture etc.
A sticky EGR valve is also a favourite for stalling type symptoms and can be difficult to diagnose, normally more a problem for diesels than petrol engines because of soot (I'm not sure if most petrols have an EGR valve?).
Your car is might be a turbo which makes it more difficult to diagnose.
Too many bits on pieces on cars these days for emission control etc - this is far too modern for my knowledge. Try asking on one of the Astra or Vauxhall forums, most Vauxhall forums are really good.
