Steering pulls during turn

I'm at a loss here. My 1990 Sable drives perfectly fine at city speeds, no problems turning. When I get to highway speeds, she cruises along just beautifully - until I steer into a curve. While I'm in a curve, it pulls and lets go, pulls and lets go... It's done this since I bought it two years ago. Since it doesn't vibrate or pull when I'm driving in a straight line, I don't think it's an alignment problem.

I've replaced both half-axles, put brand new front brake pads on it, and checked the steering linkage for worn parts. I don't see anything obvious. The power steering fluid level is fine, and it doesn't make any strange noises.

Any suggestions?