Take the car to a steering place, throw them a few dollars and ask them to look at the problem.
At least this way you know which component is failing, and you don't replace say a pump when the relief valve on the rack is dumping or similar.
Wish I had a dollar for every time someone has replaced a rack or pump and teh problem has persisted!
And be sure to thoroughly flush the system (and clean teh filter in the reservior) with fresh fluid, with the new component NOT CONNECTED, before you put it back together. No point pushing all those bits of junk through the new parts.
Regards, Andrew.