Set RT to Unlimited RT.
Set Playback Frame Rate and Playback Video Quality to Dynamic.
Close every other app while you are in FCP.
Go to Final Cut Pro > System Settings and then to the Memory and Cache tab. Pull down your application memory to between 85 and 90%. This is a counter-intuitive move, but seems to give the OS a little more play, and makes my systems run a little smoother.
Does that help?
HDV is tough on systems because so many of the frames (14 out of 15 in most cases) are virtual (not I-Frames.) If you are mastering to DVD, you might consider editing in DV anamorphic instead of HDV. Most HDV cameras will do this down-convert as you capture, or you could do the conversion of the captured files in your computer.
Here is a nice article by Larry Jordan
https://www.uemedia.net/CPC/hdvroadshow/articles/printer_13088.shtml
Here is a tutorial on one way to do the conversion
https://www.lafcpug.org/Tutorials/basic_hdvxdv.html
Here is an article from Apple on the same topic
https://docs.info.apple.com/article.html?artnum=302965
Use Google and do some research and you will discover a work flow that will work for you.
Best of luck.
Let us know how it works out for you.
Tom