Subj vehicle has 39K miles. About a week ago the power steering took on a noticable hardness but it isn't reproducable on demand. It will do this every day, and it does so when first moving (hot or cold), and then disappears after forcing the wheel, but may recur immediately if one stops or slows such as when maneuverning into a parking space. It has never done this before. Fluid levels are normal, there are no noises from the power steering, and belt tension appears to be normal. Any ideas?