I spoke to my local computer store tech guy. Explained the C: ok to capture and the internal D: dropping frames (and the USB 2.0 external also dropping frames). He suggested that the D: drive - the older one on my computer - might only be 5400 speed, while the new one is 7200 speed. He said to buy a new D: and try it. If that's not the problem, just return the drive.
Well, it worked. No dropped frames on my new 120 gig internal D: drive.
Many many thanks to all who offered suggestions.