I really have no words of wisdom, but I am wondering if anything is different besides the computers. Or is it just the operating system that's different. In other words, what I'm trying to get at, badly, is what exactly has changed. Is it a different version of Imovie? And could it be the firewire cable itself?
I've had video not import too awfully well -- now that I think about it it's been to the Powerbook, and not my desktop.
But - I have no solution for you. (Just didn't want you to think no one cared!)
You might want to look on the Apple discussion boards, too.