It could simply be due to it being an xvid file. Editors, Premier 6 especially (I know, I used to use it,) hate divx/xvid files and will often crash or just not work right. The fact you've been working fine up to now I find to be a miracle, but sometimes they do happen.
You might want to check that you can view these files in windows media player or media player classic (not VLC as it doesn't use directshow filters like premier does) to see if that is the cause.
Finally, I recomend that you simply load your AVI's into AVS scripts with avisynth and then use the AVS files in premier using the bait&switch method to replace the files. Then you will be sure it isn't premier hating your xvid avi source.
Actually, if I ever edit with xvid or any other compressed source I normally load into premier with an AVS script anyway even though my version of premier is known to handle xvid better then 6.0 did.