You've done everything right: you've tried different apps, you've enabled DMA etc etc... In these situations I'm normally the kind of person that goes the whole hog and formats and re-builds a PC. But before that, lets clucth at some straws

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With your setup, there should be no dropped frames. Could I ask a dumb question? You are definately using your firewire connection and saving to your harddisk as a DV encoded AVI file? It's not an external capture card is it? Do you have any peripheral attached (external harddrive, printer etc) whilst capturing? Have you tried another Firewire cable? Could you test your camcorder on a friend's PC?
I'm REALLY, REALLY clutching at straws, but is it a PCI Firewire card? Have you tried uninstalling and moving to another PCI slot? You might also want to pop down to a PC retailer and buy the cheapest FireWire card you can lay your hands on - (if you have the same problem, just take it back)