1998 Chevy S-10 power steering problems

After the truck had over 120.00 miles on it I had to replaced the power steering pump 3 times. Each time I had the same problems. With a new pump I have a hard time turning the steering wheel left or right, but once it was in motion I had no problems. This last for about 2500 miles. Over the next several thousand miles my steering goes from hard to almost impossible to turn when stopped, if it's at a red light pulling into a parking spot, or at a dead stop. Then the truck starts to sway side to side when I drive it. If a burst of wind hits the truck from the side it takes every bit of my strength to control it's movement. And once the truck starts to moan & groan, and it is getting too hard to control I replace the pump. Three different shops, many different mechanics, and three different makes of pumps. Two shops stated, low pressure. However, when the pumps were replaced the problem did not go away. What is being overlook? What is not being corrected?