[Deliberate2]

Friends, my wife drives an '11 35i with 111,000 miles. The car just developed what feels like a subtle hesitation under load. There appears to be no discernible loss of power. And, again, only happens on harder acceleration, not when coasting or low acceleration, and is not constant. Just had the water pump and thermostat replaced. When we did the shop suggested replacing the plugs - but as a maintenance issue, indicating that the replacement should be done at 60k.
Diagnostic thoughts? Much obliged.