I figured this out. Nubus cards have user (well, programmer) installable Interrupt exception handlers and "follow up" code that you can attach to a slot or sRsrc structure. Mundane activities that are related to the Nubus cards themselves can "steal" interrupts. Most Nubus activities are only interruptable by an NMI (Level 6 interrupts, or so it seems according to MacsBug) and apparently, these routines don't like to be MacsBugged while the 68020 is writing to Nubus Super Slot space. Either that or the actual level 6 interrupt routine is doing something funky. In any event, the only Nubus devices present in the II are the 8 bit display card and a SCSI controller, which I removed, and the problem goes away. So it must be something in this cards ROM because there are no drivers installed for it and I know Nubus ROMs can contain executeable code. Arghhhh.