Mitsubishi Repair: Power Steering, 1993 mitsubishi diamante, mitsubishi diamante

Question
I have a 1993 Mitsubishi Diamante ES, automatic.  Upon initial start-up, the steering system seems to work fine, both while moving and stationary.  However, after driving for about 20 minutes, allowing the system heat up, steering becomes difficult for sharp turns, but only while at engine idle (such as braking or coasting around a corner).  As long as the engine is reving, even a little, steering is OK.  If in park at this point, steering is very hard to turn, but slightly reving the engine brings steering back to normal.  System has fluid.  When steering is hard, you can hear the steering system whine (strain?) when turning.  I don't hear any squealing sounds.  Any thoughts?

Answer
It is possible that your power steering belt could be too tight (causing a strain on the pump), the power steering fluid has lost it viscosity, or a bearing in the power steering pump is worn.  

The power steering pump gets its ability to turn from the power of the engine.  When the engine is revving, the pump will have more power available to it, but at idle it will not have as much; though it should still be easy to turn the wheel at idle or coasting.  The reason it may seem fine when the engine is cold is because your fluid is thicker, and provides greater hydraulic pressure.  As the engine heats up, so does the fluid.  The fluid becomes "lighter" and may not provide the force needed to assist in steering at lower RPMs.

The easiest and cheapest solution is to have the old fluid drained and fresh new fluid added.  If that doesn't help, they belt should be replaced.  In fact, if none of the belts have been replaced in the past 60,000 miles, it's a good idea to do them all at once.  If that does not help, you may need to have your power steering pump rebuilt.  It could be a worn bearing.
Good luck!