Did you ever find our the reason for this?
I bought a Spider from a non-Alfa garage, and the passenger window had the same symptoms as your 159. The garage have been great at trying to fix it, but they're getting stumped. So far the drivers side switch has been replaced as well as the passenger side regulator. They've ran out of things to replace now though, and my car has been with them all week whilst I drive around in a courtesy car.
It seems to be the same symptoms as you, the window only wants to go down regardless of whether the switch it pulled up or down. Both the passenger and drivers door switches are the same, and then randomly it fixes itself and operates normally. Needless to say, the window reset procedure has been carried out several times. Is it possible that the reset procedure doesn't work reliably and needs to be repeated until it cures things?