Hah! Interesting... It looks like you found a bug with Z3 addressing.
I'm not much of a deep-level hardware guy, but it looks like any time you have a border condition, (like having the end of the RAM board at 48000000-4FFFFFFF run right up to the one starting at 50000000) it is causing problems. When there is extra space there, (like the board at 48000000-4FFFFFFF and then empty space until 58000000-5FFFFFFF) everything is ok.
Moving around the PIV seems to move around where the physical addresses of the memory ends up, allowing you to get those breaks between banks. (Or, of course, removing some RAM from the board, also gives you that break between banks, allowing the system to work.)
