You seem to be doing all the right things to get good capture.
Your video tape is good as you have tested on another system.
During capture are you getting ANY Dropped Frames
I can only assume you have a faulty Firewire Card / Cable / Connection or Driver.
There was a similar posting recently ( I cannot find at the moment)
To cut a long story installing another Firewire driver cured his problem.
Found it( not the same problem but Firewire related)
Read TDee last posting
http://phpbb.ulead.com.tw/EN/viewtopic.php?t=7698
Trevor
Hope this Helps