Artifacts occuring 9min in while compressing to mp4 using Episode 5.3

I have 5 videos I have exported from FCP6 as ProRes and importing to Episode Version 5.3 (23597_20091019). I am trying to compress to mp4 but limit the average data rate to 700 Kbps. Somehow, I get a blocky artifact occuring at the same time in all videos, about 9 minutes 6 seconds in. They look good up to that point. I have tried the mp4 template with no changes and it seems to work OK. I am currently testing an uncompressed source into Episode but don't have results yet.
Any thoughts or suggestions?