I had/am having the same problem with my year 2000 GS 300.
First thing was to install a filter in the return line to the reservoar, that removed the uneven power steering (and protects the rack in case of pump disaster).
The core of the problem could be in the solenoid at the P/S pump, however it is not sold separately. It is getting better if you excersie the solenoid by disconnecting it and apply shortly 12 V directly to the solenoid, there is a connector at the cable down at the side of the engine under the P/S pump, apply 12 V for just a second, several times, change polarity each time. You should hear a clicking sound from the solenoid, otherwise, it is stuck.
An improvement for all GS:es before year 03, is to replace the P/S control module under the dashbord and install the one for the 03, it provides 0,5 - 1,0 V more on the solenoid. Of course, won't help if the solenoid is completely stuck
89650-30610 is the part number for the ECU out of the GS Sport Design in 2003, price 278 USD. That is an improvement for all GS:s, also when your car is not having a problem.
Further improvement can be expected by replacing the P/S fluid with "Red Line Power Steering Fluid", or similar compatible high performance fluid.
If that doesn't help you have to go for a complete new P/S pump, very expensive. The rack is probably ok, if you haven't been driving around with a P/S pump that is completely run down, then all the metal fragments would have ruined the seals in the rack.
Good Luck
Rager