My 75 does this if the fluid gets low. Once its warmed up a bit and you've been using the steering its OK. Maybe some air is getting in, and bleeding out with use. I know the latter shouldn't happen, but it does all feel wierd. Once its topped up its OK again all the time.
I lost my power steering after driving through a rather large puddle once - we had what they call a hundred-year rain event last summer(!) which meant there was a lot of water left on the streets. The steering was unbelievably heavy!