As Rafael suggests, you should look for a non-AVI workflow. You are running into a 4GB file size limit for AVI export from Final Cut. That is why short AVI’s work but not long ones. AVI is really an outmoded container format for computer video. With Quicktime there is no such limit, although you will have similar trouble if you try to export to a network drive or attached drive that is formatted in FAT32 format. Again, there is a file-size limit of 4GB.
A possible workaround would be to export a Quicktime movie first and then use Quicktime Pro to change that to AVI on a PC system. But really, avoid AVI altogether if you can.
Final Cut Pro 6.0.5, Mac OS-X 10.5.5, Quicktime 7.5.5, Adobe Prod Prem CS4, G5 Quad 2.5, Kona-LHe V6, 8.5GB RAM, Nvidia GeForce 7800-GT 256MB, G-RAID 2x1TB FW800, 6TB RAID-5 (Enhance E8-ML, Highpoint 2322), Panasonic HVX-200P P2. Also MBP 17″ Core 2 Duo 2.5, 4GB, GeForce 8600M GT 512MB.