I have a horrible suspicion that ICS sharing is broken in XP, at least with wireless. It attempts to create a layer two bridge, to avoid routing. However, for some odd reason it then decides that the 2nd (connected to router/internet) NIC should have a seperate IP address, and that this IP adress should be the default gateway. And more often than not, it decides to assign the same IP address as the existing router....
This is plain daft. TBH, better to bite the bullet and use a wired connection from Amiga to router. Tried, tested, and works.