so is your car just losing power? does it feel very sluggish and like it's running off 5 cylinders?..we've all seen those beaten up VT's going around where the driver plants their foot and the car just goes brrrr, and goes next to no where...i've worked on a couple of ecotec where they've had valve springs broken..there's nothing wrong with the engine, its just one cylinder fighting against the valves and compression therefore losing power and feeling sluggish...so yea if you can, or get a decent independant mechanic to, just check your valves and springs and camshaft wear etc
if everything seems ok there, get a compression test done...at an indepedant mechanic with a reputable name..if you ask them to diagnose something, they'll do it right for you so they can get your business to fix the problem..get them to do a wet and dry test..just incase you don't know i'll give you a little bit of info on it.
dry test- spark plugs are taken out and a pump like device is scrwed into the spark plugs place...they put pressure into the cylinder to make sure it's holding compression..if it holds pressure, it's fine, if not, it could be a number of things that won't allow it to seal eg, piston rings, cracks in the cylinder, bent valves or they aren't seating properly etc...leave the mechanic to diagnose whats precisely wrong.
wet test-, they will put oil into the cylinder..gravity will cause the oil to drop straight onto the piston and around the rings...they do the same thing as a dry test now, put pressure in it and see if it holds pressure...if it holds, it's almost guaranteed it wil be rings..seeing as though the oil will help form a seal around the rings, therefore, better compression.
well thats just a bit of basic information about it...and what you could be up for...but don't trust holden, i've worked there and i've even been asked to do a bodgy job for a customer just to keep him happy...i got into an argument koz i didn't want to do a dogy job...independant well known mechanic..maybe even a performance place will be a good place too.
if you can describe to me what exactly and every single symptom you can tell me..any observations you've had, just tell me..i have a bit of handy info(service manual) that may help you out to diagnose it.
cheers mate