Sorry to hear about your problems, the dealer is fobbing you off with the 'fuel contamination' excuse, it sounds like you have a genuine ongoing misfire.
Do you know if they've checked the basics such as the spark plug condition, coil/injector/sensor connectors are engaged fully, wiring loom condition (no chafing), fuses/relays are all seated properly, earth bolts are tight? If the engine is misfiring it's due to incorrect fuel/air mixture, a faulty component or connection. You could remove the plugs yourself and have a look at them, if one is obviously a different colour/condition then that tells you which cylinder has the problem. Try replacing that plug. If you're getting no joy from the dealer you could do quite a lot yourself to help diagnose the problem just by some elementary checks under the bonnet. I would start by checking the connectors on the coils and injectors (check they're 'clicked' on fully and check the pins aren't pushed back inside the connector), check the plugs, the coils are fully engaged on the plugs (with the plug and coil removed, ensure the coil engages properly with the top of the plug).
Hope that helps!
cheers
John