AJA makes extremely reliable gear. It will work as advertised, and more, but frankly FCP is a little doggy and specific with it’s technical menus and settings. You’ll need to provide us a lot more specifics on your system before we can help.
It sounds like your system isn’t set up right at all. Was this a home built system, an order from a video reseller?
are you running leopard? Tiger?
What are you trying to digitize at? DV25? HD? 720? 1080?
What are you sequence settings? What is your Video out setting?