The X-surf had worked OK for a long time, but about a year ago it stopped working again. So I bought an X-surf-100 and everything is OK in the A4000.
I also have an A3000 and I thought, why not try the broken X-surf in there? It used to work in an A2000.
And it works flawlessly. So the problem is definitely not in the X-surf, but in the combination with an A4000D/Cyberstorm 060.