New spark plugs now power surge

Why aren't you bringing it back to the dealer that did the work if they caused the issue?

The dealer cannot identify the problem

That's a ridiculous answer. It is their responsibility to find the problem and if they are a new car dealer, they are the ones most trained to do that. We don't have that car here in the US so we have nothing specific to that vehicle. Any competent technician should be able to narrow down the issue using diagnostic equipment.