15GB is more than a little unusual for 720x480, regardless of codec. A 720x480 (3:2 or 16:9) DVD, which must be encoded with MPG-2 tops out at 8.9GB. That's pushing a lot of data for its dimensions. An hour of 720x480 in presentation format (as on a DVD disk) is lucky if it hits 2 or 3GB.

Videos get big like this when encoded with a lossless or 'visually lossless' (i.e. Cineform, DNX..) intermediate codec, which is great when multiple codec passes is needed, but doesn't help any at all when scoring. 
 
It is interesting that that X2 is working better with that file. It might be that changes in X3 do not allow such large caching as would be required that file. I'd stick with X2 for now - deadlines are deadlines. But I'd still have point out that it's far better to score to proxy files, they're nice and small and they don't get in the way.
 
Cheers!
 
 

did you get a message that the the wavefors were being calculated? I have had projects that did not work right until the process completed. You could also increase the size of the PicCachMB setting in the configuration file. It controls the size of the cache. 

SuperG, you're right it's a big file, but the person doing the rendering is technically unsavvy so I havent been able to get a happy medium of quality versus size (nor had the time to re-render the file before having to record...so it was a good thing X2 fired right up a couple of weeks ago when it had to). However, it doesn't answer the question of why I am still getting the problems after deleting the video, so until  otherwise informed, I am going to assume/believe that X3 needs to re-write (for lack of a better term) all of crossfades, slip edits, and other clip/track data, because Saving As... to another folder and saving the clips with the session as new clips fixed the issue.
In fact as we are done with the ADR, I'm going to re-render the AVI  and see if that helps.

Tom Riggs: no warnings/pop-ups that anything is being done other than the HDD lights having a ball.
 

Oscar, I know you're busy...
 
Just trying to help. Let's us know how the re-render goes.  
 
 

I kinda had the same problem with opening an x2 file in x1 on a different computer. X1 seemed to open it OK even tho I got the expected message about some features may not work properly, etc. However, when I tried to record some stuff...drop outs every few seconds! But when I copied all the tracks to a new project and tried recording again when the new project was saved, all was well.

Allllllright: the original file is working now. On my break [Bwahahahahaaaa: break!] I mucked about with the original X2-to-X3 file. I believe my hypothesis to be correct : the number of clips was taking X3 an inordinate amount of time to make the X2-created file into an X3-friendly file (Bakers want to respond? Am I off track?).
So I can be happy now. Are you happy? I'm happy. 
 
SuperG: I appreciate your knowledge, I just wanted you to know how I got where I am on this. 
 
Again, thank you everyone for your assistance. Now: to really screw things up and actually start mixing and Foleying (in the audio sense, not the medical sense...ew) and adding Perfect Spaces and Compressors and ...  
I'm going to see what X3 can take!