Is this DPF regenning, as phwspider says? Sounds like the dpf is pretty blocked, and/or not regenning propely. This is exactly the same as I used to get. I think my cars had mainly been used for short journeys before I bought it - and despite me using it mainly for longer and motorway journeys, and giving it the old italian service from time to time to clear its tubes, this problem kept recurring. Even getting forced regens done didn't really help, and the wavering became a noticeable judder.
Odd that you mention the turbo pressure seems smooth, though. Typically on regen, everytime you take your foot off the accelerator, the turbo needle would drop like a stone.
A new alfa dpf is extortionate in cost. There are some reconditioned units that come up from time to time. If you want to keep the dpf, get it drilled/cleaned. I got so fed-up, I got a full egr blank/dpf delete and map - much better performance, better mpg too.