Unknown error on Media Encoder

I'm trying to render a movie and every time I get about half way through it crashes saying unknown error..

Oddly I can render the 1st half of the movie and then quit and restart and render the 2nd half of my movie, but not both together..

I've tried reinstalling CS4 (totally clean removal and install), duplicating clips and reimporting them, creating new project files and importing old sequences, etc. Nothing seems to work.

I'm using CS4 (Premiere 4.2.1 and Media Encoder 4.2) on a Mac Pro with OS 10.7.5

anyone have any ideas?