My Solution: I use Windows XP and a mini DV (tape) Canon HV40 camcorder that is set to record and output in 1080 high def. My jerky or choppy captures were driving me crazy. What is happening is that frames are being dropped during capture. Its my conclusion that my PC needed to devote its full attention to the capture process due to the huge amount of data being moved to the hard drive. When I stopped my Antivirus. Malware protection, and Registry Monitors, it made a world of difference.
And I don't mean stopping scans during the capture, I mean disabling or shutting down the whole application for the capture time period. I also stopped a few iTunes apps just in case they tried to access the Internet. Finding the applications that automatically use CPU time or access your hard drive in a background is the key. I have a 2.4GH duo core Intel with plenty of free hard drive and Ram Memory, so I don't think my PC is underpowered.
I might also add that if I set the HV40 to output in std DVD 720 and used something like WIndows Movie Maker, I had no problems with capture while the antivirus was still enabled. Maybe lucky, but my hunch is that its due to handling a much smaller amount of data.