BMW - 325 :: Steering Sticky On Curvy Roads At Moderate Speeds

I have a 2003 BMW 325xi wagon with mileage of 56k. In warm weather and when the car warms up, on roads with lots of alternating curves, at moderate speeds, on some right turns, the steering will offer a slight resistance and then give. On left turns, there is sometimes a slight loose feeling and a corresponding resistance when I straighten out after the curve. This intermittent problem occurs rarely in a parking lot, as if the workout on a curvy road is necessary to cause the issue. When it occurs in a parking lot, the "give" after the resistance is accompanied by an audible "thump". This noise presumably occurs all the time but is masked by the noises of the car in motion.

The dealer in my home town could not recreate the problem until the visit where I took the mechanic for a ride and the thunk noise happened once. They said the steering U joint was the problem and "Replace steering u joint and filter housing gasket and vanos oil line. Perform power steering flush".

After this work and a trip to the mountains, the problem seemed worse, with the issue spreading to left turns. I complained to my home town dealer, who offered not much more than his sympathies.

I took the car to a dealer near our mountain retreat. He said my home town dealer had not performed the power steering flush and that he recommended doing it, even though he doubted it would fix the problem. He did hear the thunk (he called a pop) when the car was on the rack, and he ruled out every issue but the power steering rack, which he says to replace. What steps can I take to ensure proper diagnosis?