I had the same problem a few months ago. Replaced the worn rubber power steering bushings with urathane and tightened the belt. I could look at the belt alignment before I started and the pump was canted forward. No problem now!

This symptoms sounds like low power steering pump pressures. Like the prior post check the belt tension. If you have a weak pump, you may experience the stiffness at a low speed while turning. The problem goes away at higher engine speeds. I have not seen the problem on cars but it is common on semi trucks with Luk pumps.

/// Check the steering pump belt. It is non critical and is often just adjusted until it rests in the bottom of the pulley sheave. At that point the belt cannot function. Look for polished bare metal in the bottom of the pulley.

If your problem were the yokes in the steering shaft you would experience trouble almost every time you turned the wheel instead of at the extreme ends of the steering range. Have some one turn the steering wheel while you watch the yokes.

Belts have a fixed outer diameter due to the stiff backing band. In a straight section the bottom, "V", and top, backing band, are the same length. When bent into a half circle around the pulley the inside or bottom of the belt must expand toward the center of the pulley because of the difference in the radius of the two arcs. If that cannot happen the belt will slip when the load increases, at the end of the steering range.

Try lubing the U-joints in the steering shaft. Just squirt some oil on them.

Could it be your steering yoke in the shaft? I recently had to replace the lower yoke as it was stiff. All my steering is back to normal now.