power steering is greatly diminished at low engine idle

Check the tire pressures first. If the power steering belt is loose steering pump pressure will be diminished but you would probably hear the belt "slipping". Check the fluid level (but if it is low the pump "growls"). It is most likely the pump itself has a problem but there are other possible problems, something in the suspension or steering could be seizing or binding up, the steering rack could be bad or a steering hose could be restricted.