I don't have any experience of the late C5 but I have had a couple of the earlier ones and still have one. This pitching from side to side I have had with both cars but mostly on really rough road surfaces and I think it as because the anti roll bars are to thick. I ran XMs for 15 years and CXs before that and they both had a better ride than the C5 although by modern standards the C5 is good.

I have replaced all the spheres on my current one and although the ride has improved the pitching is still there. If the spheres are good you should be able to bounce each corner at the rear about 4"- 6" and at the front somewhat less but at least 2 - 3".

I'd have said five years/50k is about right for sphere life.

Just got rid of my 2000/X Xantia estate.Original spheres were replaced in 2007-8 and replacements were completely shot. Front and rear so firm that ride was dangerous. I'd foolishly ignored a couple of instances where it had handled oddly under emergency braking. When I finally took it in garage gave me strong advice to nurse it home and not drive it again. New car was already on order and Xant had other problems such that it went for scrap.

Best replace spheres when they start to go west as they can be exchanged or re-gassed.

Let them go to far and they're scrap.