Today after work I started my Silvia up and I noticed that it was idling at 1.5k rpm not the usual, but I put that down to cold start. When I was driving, now and then I would hear a whining noise which eventually stopped. So I pulled out of a junction onto a 60mph road and booted it to 60, I then approached a bend where I found my steering went stiff and nearly crashed into a wall. I managed to drive home, very pale, but the steering was so stiff it was crazy. I got back and checked the fluid and I have a full pot. Any ideas why it would go like that?