Enter Question here:analog to digital transfer onto iMac

I am using NTSC-analog video player running at V110/ 60hz and a
PYROW A/V LINK with an firefire output connected to my iMac. I am trying to transfer analog footage on the the iMac using the iMovie program. I get a connection , but in the transfer progress it cuts
out for few seconds and then it startss again - cuts out - starts again etc.
What am I doing wrong? How should I configure my system.
Anbody who can help me?
sincerely
Jurgen Toussaint