I too had this same issue about six months ago. I had a split pipe as well as an old nacked MAF sensor. I replaced the pipe with a genuine one from Alfa (£££££ Ouch) and bought a LUCAS replacement MAF sensor from Panks. It worked fine to start with, WELL, what I thought was an improvement!!
The way I noticed it wasn't working correctly was by fitting a genuine Bosch part from a Vectra V6 believe it or not. They are the exact same MAF sensor but are given different codes as the plastic piping part that comes with them are different. Anyway, tried it and it ran much smoother with less idle movement when coming to a stop. The major difference between the two was on re-application of the throttle from letting off whilst on the move. When re-applying I would get a 'judder', as if the engine wasn't able to assess what to do quickly enough. Anyway, all sorted. Stick to Bosch if you can.