Most of the problems with DV or HDV capture are based on latency, not throughput. Given that you have DV25 captures that are flawless, your PC is certainly capable of capturing HDV (same bitrate) just as well.
So it's probably the capture program. I assume you've been using Vegas Studio. I was never happy with Vegas Pro's built-in capture. So, back in the HDV days (some years ago), I used a program called HDVSplit for capture. See here:
http://strony.aster.pl/paviko/hdvsplit.htm
The other issue could be your OS... there are sometimes conflicts on 64-bit systems with 32-bit drivers, or software not tuned to properly use a 64-bit driver. Or lazy driver or CODEC ports (though the latter probably not an issue in this case). Anyway, worth a try.
-Dave