in general the power steering pump doesn't produce enough GPM or PSI at an idle when you are trying to turn the wheel and/or have your foot on the brake, cause it supplies power for our power brakes. there really isn't much to do about this except use quality synthetic fluid especially when it is cold outside and try not to crank on the wheel when sitting still... that by itself is the hardest thing that anybody can do to all the steering gear... box, pump, linkages, tierods, etc.