A mystery to me
 

I am working on an eighty film thus have divided my project into 15 sections to make the work flow more managable. As an animator I create all my clips within the computer so there is no camera and import of footage involved.

I import my clips as 720p and have pretty much absolute control over their unity of quality and size and the same goes for my sound tracks. I use no special effects in my Vegas Pro 8b projects besides automatic dissolves. I can play back my timelines at a consistent speed of 24fps with Best/full preview window at any size of the window I wish. I can view my projects on a secondary screen which I place on a third monitor at full screen resolution with no loss of playback speed.

But recently the new segment I started wouldn't perform like this pretty much from the beginning, or let's say after I had assembled about 2 minutes of video clips to a 10 minute sound track. To be able to run the timeline at a consistent speed of 24fps I had to make all sorts of adjustments such as diminish the size of the preview window, reset its quality down to just good/full etc., and as my project grew to about 4 minutes even these measures wouldn't help much.

So I copied the entire project which consisted of two video tracks and two audio tracks and a bunch of volume level envelopes, opened a new timeline, pasted the copy into that and all runs well and as expected. Would anyone here know what could have been bogging down my playback speed? I've had this happen before and just lived with the inconvenience, never thinking that I could do the copy and paste trick until now.

On the surface, i'd guess that a resource wasn't released/leaked. It happens from time to time, particularly with one of two instances:
A-you've got lots of small few frame/seconds files (hardest for Vegas to deal with)
B-you've got a single, very long file.
Either have caused choppy playback for me from time to time.

Thanks, Douglas. That must be it then. This segment has many 3 to 4 second scenes and one 30 second one. But in that case what happens when I just pick the whole lot up and paste it into a fresh timeline, which fixes the uneven playback? Does just the fact that they are imported all trimmed and at the same time clear something?

By the way, my clips are uncompressed (BGR24) AVIs. Would switching to a different codec make a difference? QT Animation (lossless and as I understand, the equivalent of uncompressed) was terrible. Anytime there was a camera move in the scene the playback would become uneven and slow down.

Would it perhaps be a good idea to import my clips as image sequences?