First check the usual things: Terminations properly located and functional? Power supply stable (measure at drive terminals)? Connectors (inside and out) clean? Unique SCSI IDs for all devices in the chain?

If all that checks out, I'd try using a hard drive tool that does a thorough analysis of the drive and automatically reassigns bad blocks. HDT and SCSI Director are a couple that come to mind.

If the drive still won't play nice, it's time to retire it.