I have the same problem when I work with longish vids: at some point, Premiere just doesn't let me import any compressed avi snippets any more (they show up as audio files even though they don't even have an audio track); the only files it lets import correctly are huge uncompressed RGB/YCbCr files. This has happened to me with two vids I am working on, both were close to finished with around three and a half minutes of timeline filled when that particular problem occurred. Restarting Premiere or the computer didn't help, only resporting to the above mentioned uncompressed avis.

So, yes, I have the same problem, unfortunately I don't know how to solve it, only how to work around it, which of course requires a lot more space.

(My specs: P4 3 GHz with 2.5G RAM, WinXP SP2 and Premiere Pro 2.0)