Have you tried installing different drivers? I had problems with AJA IO v3.0 (with my OS and FCP versions) and went back to v2.1.
May not work but might be worth a try.
Thanks for the replies, big help. It was a driver problem.
Version 3 driver doesn’t work with FCP 4.5, so had to go back to Version 2.1 driver.
Regards
Rod