I have a very similar problem on my 1.8ts. It seems to be worse when the car is cold and becomes much less noticeable after five or so minutes driving.
After my usual mechanic had no luck finding the problem I took it to an Alfa speciallist in Keighley (C.W Lear 01535 664559) and he said there were actually two problems.
The first was that my lamda sensor was duff (this was spotted with an ECU reading computer). The sensor was changed although it has made the car run much much better overall it still hasn't totally cured the flat spot problem.
The flat spot problem he thinks is caused by what he called the advanced cam variator. Unfortunately he didn't have time to look at it that day so the car is booked in a week on monday.
This is how he explained it to me (not technical)
At a certain rev. rate a solenoid is fired by the ECU. This opens a valve that lets oil flow though a hole and this 'advances the cam'. He thinks the problem is either dirt in the hole or the solenoid itself.
Another reason he believes that the cam variator is the problem is that if you slowly press the accelerator (while just sat with the engine running not actually driving) the revs will almost hit 2000 then 'bounce' back down like you have hit some kind of rev limiter. He said this is occuring when the ECU is sending the signal to advance the cam and it not happening properly/fast enough/at the right time.
Anyway, he hasn't actually looked at the problem yet so this is all speculation. I will update this post when he has done the work.