I would check if all the interrupt handlers you have do either not set PPAGE/DPAGE,
or they do properlty back it up before modifying.
The descriptions sounds like if PPAGE would get corrupted by an interrupt,
do you "loose" the target when you are stepping in the 0x8000..0xBFFF PPAGE area?
Maybe try to assembly single step and try to catch the case where it just jumps away.
Daniel