one more thing...if you are starting the PC from an IDE (EIDE) drive, go into SCSI BIOS and delay spin up of SCSI drives...makes them suck power well after the intialisation of motherboard and other pieces...test the system..looks like you know what you are doing, disconnect whatever you do not need and try the zip drive, than add again...i still guess it is a power problem...
forgot something, if you have a lot of USB devices connected, just disconnect them and tell us more...