Volvo s60 power steering problem

I have a 2003 volvo s60 that is hard to steer while driving straight down the highway. When your driving around town it’s fine but when you drive it straight for a few minutes the steering becomes very stiff as if the power steering is broken. When you do turn the wheel there is resistance at first but then the power steering kicks in. I took it to a Volvo dealer and they said there is a valve in the steering rack that gets stuck and I’d need to have the steering rack replaced. They quoted me 1900.00 to do the job so I decided to shop around and found a highly recommended independent mechanic who said he could to it for around 1100.00 . I left the car with him a couple of days ago and got a call saying he had replaced the steering rack and it didn’t fix it!
He said he was perplexed. Does anyone have an idea on what else the problem might be?