I have been having this issue since I started editing HD & upgraded to Pro 9.0 (haven't tried any other versions for HD)
While I'm playing back my timeline, I can play it for about 30-60 seconds in "Preview - Auto" mode before it starts getting REALLY choppy, then I can switch to "Draft - Auto" and get about another 30-60 seconds. Then switch back to "Preview - Auto" (or better quality if I wanted) and repeat this as long as I need to.
I KNOW my machine is capable of playing back the video it's a quad 2.4ghz with 8gb of memory running on 64bit Windows 7
This issue persists with SD footage as well, it's not an HD issue.
I'm hoping this is a known issue with a work around!