Ron,
Sorry I don’t have an answer, but I am having the same problem when exporting AVI’s from FCP 5.1.2 to an external drive. I can export 5 minute AVI’s & they play fine on a PC in Windows Media Player, but like you said anything in the 30+ minute range gets strange. On a PC I can get a 30+ minute clip to play in Quicktime, but not in Windows Media Player. I also imported the AVI back into FCP, and even though it was not optimized for FCP, it played back in the viewer. We may be missing something very basic, but I hope someone has some insight.
Best,
NathanH