There are three things which effect the power steering system. Firstly faulty power steering pump, faulty high pressure flexible supply/delivery pipe and lastly the rack and pinion known as Kangi. If power pump pulley is unbalance then better change it or get it checked from a mechanic. In my opinion your car kangi is being repaired and when its rack end bushing and steering rod bushing are changed then the steering become hard. However, there is a adjustment screw in kangi from where you can vary the stiffness of steering. Also check the condition of power pump belt, sometime the belt is slipping and as you said that steering becomes hard after engine warm up that means the belt become soft on heating and it loosen up and starting slipping on pulley and thus steering become stiff or hard. Try to replace the belt with new one with recommended size. Never over tighten the belt as it may damage the bearing of pump. There should be at least 1/4" slag in belt while pressing the center of belt with thumb, it is important for the safety of pump.