Power Steering Problem

[stumsport]

Hi
I am new to the site and would be grateful if any one can help me. I have a problem with my steering I have recently fitted a second hand rack to my car which was in perfect working order but over the past few weeks its been getting harder to turn left, like there is no power from the pump. It starts off normal for about quarter off a turn, then suddenly feels like its catching something and gets very hard to turn. If I turn the wheel very slowly its fine but under normal driving the problem is there. There is no problem with direction i just point it to where I want to go. Can anyone help its getting on my nerves. Also how easy is it to fit an E46 steering rack any ideas?.

Cheers

Stu