Problem The steering wheel goes hard sometimes


At certain moments, which I cannot relate to the temperature or other conditions, the steering wheel turns hard, as if I had no power steering. This is combined with a noise that can sometimes be heard at the maximum travel of the steering wheel (a squeal). At the service they told me that it is 100% the power steering pump, which should be replaced. The problem is that sometimes the power steering works normally, the steering wheel turns easily, and the noise does not appear at the end of the left or right stroke. When the steering wheel goes hard, if I accelerate to 2000 revolutions, the resistance of the steering wheel eases considerably, the aspect being explained by the mechanics that the power steering pump does not put enough pressure at idle. They checked and there are no signs of running on the box and the belt that drives the servo pump is stretched. I checked the liquid level and it's ok, now approx. 2 months was below the minimum and I completed it, about then the described problems appeared. If you have other possible explanations, thank you in advance!