This tends to be one of those faults that can be caused by 101 different things. Therefore it is a matter of trial & error to get around it. Here are a few things to try:
Another firewire cable - even a brand new cable can be faulty
Defragment the HDD - fragmentation slows down the write speeds and can cause dropped frames.
Clear down as much space as possible - DV AVI eats up approx 13Gb per hour and as the HDD fills up it slows down.
Make sure DMA is enabled for the HDD.
Stop
all unrequired programs during capture such as firewall/antivirus etc (disconnect from internet first though

). I use a great free program called EndItAll2 to do this - do an internet search for it.
That will do for now. I'm sure I've forgotten many other things you can try, but that should be a good starting point.
Mark.