Ask Smithy Xtra Holden Viva engine problem

My 2006 Holden Viva has a number of engine problems, all of which have been investigated by Holden without success. The engine will idle at high revs then go back to the revs, the engine malfunction light will come on maybe for a day or so and then not come on for months, it surges when travelling at 80-100 km/hr and these become more noticeable when travelling up a hill. The dealer has checked the spark plugs, leads, fuel filter, and reprogrammed the ECU, and has suggested using injector cleaner and better quality fuel. Any assistance you might be able to offer in resolving these issues would be much appreciated.