OK, so I've got a 2.2 JTS so a completely different kettle of fish but I had the very same symptoms as you describe.
Last September, after changing the oil and filter, my Brera developed a slight hesitancy under light throttle below 2000 revs. This disappeared under acceleration.
A few days or weeks later, I was never sure which, I noticed the illumination on the information screen (between the main dials) and the tail lights would flicker on tickover.
This said to me, voltage regulator even though it had only done 35,000 miles, so after putting up with it for a few months, my Brera is really only a weekend car, I replaced the alternator.
As expected, this cured the flickering and, surprise surprise, it cured the hesitancy also.
I am not saying this is what is wrong with your car, but it is worth taking a close look at your alternator. You just never know.