OK, so you've ruled out the belt because it's new. We didn't know that.

What pulley did you replace? Crankshaft pulley or PS pump pulley?

After ruling out those simple things, now maybe it's the PS pump itself. But the funny thing is, turning the steering while NOT moving puts more load on the pump vs. steering while going 15mph. What about suspension alignment? Maybe your toe is way off when the steering is turned. That's a more unusual alignment issue, related to bent knuckles & stuff like that.