Wonder what caused your pulley to crack? Perhaps there'd been a fault in it when new just waiting to fail....
There are many on here who know more about these engines than me, but I have read on here of racks going u/s. Is the problem worse at full lock? I'd also look at wheel alignment, warped disc as well (although that would produce a pulsing through the pedal normally).
I have the same model as well as a couple of classic Alfas. Really like the 159 and the engine seems very robust so far. Hope you enjoy yours.
Using yesterday's technology, to create tomorrow's problems, today.