I have experienced exactly the same problem. When switching gear from 2nd to 3rd, it would jump to 5th.
Alfa Romeo could not find the issue as it DON'T show up on the analyzer afterwards. It must be plugged in when it happens. The shop re-calibrated the system but it worked fine only for 2 days. My fault came up most of times in hot weather and city traffic. Finally I barked at the ALFA head office, who helped me out and told the garade that they have to fix the problem. (the garage told me nothing was wrong with the car! Nothing on analyzer
) Well that was so bad service for my taste that I called out the RAC the next time it happened. They towed me back to the Alfa garage.
Finally as in all other treads, they had to replace the famous actuator. 1080£
Most likely that will be your solution as well.
But earlier in this tread i can read that there was a manufaturing fault, with free replacing sometimes in 2001? Do i missunderstand? If I don't then we have all been totally conned by Alfa.
I still love the car
Much cooler than my past car, a Mercedes. Fingers chrossed, no brunouts, I hope it lasts.