I'd guess you've run out of system resources by frame 80. The memory leak is cumulative.

Instead of outputting an AVI, configure Poser to render an image sequence. When Poser stops, you can reboot and continue rendering the sequence from the last frame successfully rendered.

Then use VirtualDub or some other utility to load the image sequences and convert them to AVI format. VDub can read BMP and TGA files.