If the injector cleaner helped, albeit temporarily, I would say thats a good indicator of the problem. I used redex advanced on a 3/4 full tank of petrol and it made a massive difference to my 1.8t
Has it had a new fuel filter lately? A new one wouldn't hurt (and is VERY cheap!).
If the problem persists, I would start looking at new injectors, ignition coil and the MAF sensor next.