Atto cards are the traditional Mac SCSI controller cards.
However, you should make sure you are using the correct cable, with the correct termination. I use these:
Adaptec ACK-68I2-LVD-LP-Round-U320, with a TMC Low profile U320 internal terminator T68INT-PTLVD320. These are easy to install.

SCSI is not a straightforward technology, so it would help to know more about the SCSI cabling and termination you are using.

Hugh