Ok on what type of Mac?

Age , capacitors or straight up chips starting to fail or SCSI black magic could cause this.

Have you terminated one at a time and tried? Then two at a time switching termination ends?

I have 5 devices on my color classic and as long as the last on the chain is terminated there are no issues.

CDROM, jaz, zip, orb, apple external hard drive. I usually leave the zip on the end as it's the lightest and easiest to move.