my power steering seems to have a life of it's own. somethines it's as light as a feather, and others, it feels like my old non pas chimaera. (both at parking speeds)
is the likely cause a need for more power steering fluid? there's no noises that sound like a slipping belt btw.
The u/v joint normally gets stiffer and stiffer with time, a knackered steering column bush will cause it sometimes to be ok then other times stiff. Worry if it starts grinding. Or low fluid, of course.