must have missed this topic - just seen it. Mine has been suffering a "lag" on acceleration occasionally. It seemed like on one of my old engines when the carb wouldn't be able to suck in enough air. Switching to twin dual Webbers solved the problem (and gave a great sounding sucking noise on acceleration too!).
I wondered if it was an air thing on the C4.
Anyway simply letting up the pedal and then pushing down again seems to solve it for me - hadn't thought to mention it to the dealer.
Engine, Exhaust, Drivetrain, ECU