Probably there is some ghosting going on. Could you try:
- disconnect A590
- AddMem $180000-$1fffff
- see if it actually works / RAM test
Did you AddMem $300000-$3fffff with the A590 connected? (I don't think this is possible because of the way free mem is organized, but am not at all sure.)
Try this e.g. in Basic:
- connect A590
- don't Addmem
- peek($3fffff), note value
- poke something else to $0fffff
- peek again and compare value, if it's changed there is ghosting
- repeat with pokeing $0effff, $0dffff, ..., $08ffff
The best way to find out fore sure would be to open up amy and look what cable connections were added inside. To eliminate ghosting you'd have to modify the hack anyway...