Cant get a clean h.264 render out of Resolve 16b3

Using Resolve 16b3 and Ive tried 8 times to render a 10 min video as a QT h264. I keep getting scrambled blocky sections in different parts of the video. Ive even tried rendering out a DNxHR 444 10bit and re-rendering that as a h.264. No matter what I do, if I use an h264 I cant get a clean render. Any ideas? Never had this problem with v15.