Sounds to me like the upper stops on the rails have moved. These are accesed by removing the door card completely and if you face the inner door skin you will see 2 adjusting nuts in the the upper corners. It's a bit of hit n miss to adjust from personal experience!
I have a similar problem but in my case the banging noise is my head pounding the steering wheel in frustration !
My window was slightly out, the height needed adjustment, an hours job I thought. Played around with the stops and as far as I can tell this made no difference at all,
even elongated the holes a little to get more movement.
One thing led to another and I spent another four hours adjusting everything. Result - worse than ever. Not too bad when you are stationary but if you put the window down whilst on the move then try to put it up you are left with a huge gap down the rear of the window. This happens even if you inch up the window manually on the button and try to get to the compromise position that gives the best seal.
This means that another battle with this is on the cards as soon as the snow goes and it's not below zero. I did run across an ex Alfa mechanic last weekend and he said that you should first do the front to back adjustment and then close the door on the FIRST latch, i.e. not fully closed, and then adjust the angles so that the glass is sitting on the Seal. Haven't tried this yet but I will be, when I can work up the enthusiasm !!!
I think on your P2 that there is a cam that controls the microswitch, this is mounted on the Reg itself I believe. It's possible that this has come loose and that it is no longer operating the microswitch at the correct point, if at all. On cars without the Autodrop I'm also pretty sure that this microswitch is not fitted. If it is it's bluidy well hidden.