I posted twice in the initial thread in what has become a thread blizzard. My posts were ignored, and instead all these parallel posts, which have nothing new to add, have been started.
LarsHD is now blocked on my computer because he doesn't respond to my posts, and seems to simply thinks that by posting the same exact problem over and over something good is going to happen. The problem is real enough, but there are ways to minimize it, some of which I already suggested, and some of which others have suggested. The problem is a combination of how Vegas's underlying structure is designed so that it can handle any video codec, at any frame size, any frame rate, with multiple effects being applied, into an external or internal monitor, at multiple playback speeds ... well, you get the idea. It is a totally different design than an editor that only handles a limited range of inputs, or a media player that only handles playback of already-rendered video.
I'm not usually an apologist for Vegas, and this problem bugs me too (especially the past few days when I've been editing some very unusual film at oddball framerates), but the flip side is that I've been able to edit some very unusual film at oddball framerates, and actually finish the project. Amazing.
Lars, to reduce stutter is always better to set the preview quality to Preview Full, and not Best or Good. Playback also seems to be more fluid when reducing the amount set to RAM Preview, although that's a problem if you need to use RAM Preview because pre-rendering in Vegas doesn't work well.
Now, if you want a decent NLE that will handle playback much better at full HD I recommend either Edius Neo 2.5 Booster or Edius 5. Booster can edit AVCHD natively and play it back at full 29.97 with no stutter, even after having applied color correction and other filters to it. Not all filters will play in real time, but if it has to prerender it does it very fast. I've been doing a lot of editing with it lately because it bothers me having to do a RAM preview or pre-render in Vegas to simply be able to play back in real time a clip after having applied a simple color correction or even a simple cross-dissolve transition. With Neo Booster those things don't exist, you apply an effect and normally it plays in real time without even rendering first, as well as most transitions.
I don't have Edius 5 yet (the Pro version) but if it's younger sibling is any indication, I bet it's really good too.