Audition mixdown sessions are all out of whack!

I've been using Adobe Audition for years, but just upgraded to the 2018 CC version.

When I try to mixdown a session to a new file it's all jumbled, and the timing is completely off. It only does this on sessions that have quite a few tracks (more than 16), so I would assume it has something to do with my system trying to keep up?

I have a very fast system I-7 processor with 16GB ram, so it should be able to keep up just fine. It always kept up fine with previous versions.

Any ideas?