Took some screenshots to show you what I mean. The first line of type starts out rough, but 80 frames later (just before it spins off the page), it smooths out and looks perfect. I can't see any setting differences between the two frames - any ideas how I can fix this? It does render like this too - so it's not just a preview issue.
Well, let's be clear here: No point in judging such stuff at anything but 100% zoom and full resolution. AE does not re-rasterize content dynamically and hence what you are looking at can simply be lower-res preview data. And that is kind of the point - unless you explicitly flush the caches or set them in such a way that they would not be used, you may always see such odd behavior every now and then. So purge your caches and also check the respective settings for them and related stuff like hardware acceleration, the preview panel and so on.
Thanks for your reply, I understand most of what you're saying and have purged the cache (it was 182gb). Quit and re-opened AE and it still looks the same (even at 100% and full resolution). It seems strange that it's only happening on the first line of text. This happens 4 times throughout the whole (1200x628px 25fps 1 minute) video. All the other text is fine.
Even if this was down to how AE was previewing the video - would it still render this bitmapping into the mp4 (which it is)?
The renders should be fine. Just make sure to turn off cache usage and possibly also turn of hardware acceleration to really make AE render each frame from scratch. Other than that you might try and debug your comp(s) further. Perhaps this isn't in fact to blame on the text layer, but rather another layer or effect that comes into play when the text is overlayed on top. Similarly, your 3D layer switches and motion blur preview could be playing tricks on you. Check this stuff as well.
A colleague (the person that originally created this video) figured out what was wrong...
The 3D renderer was set to Cinema 4D - changing it to Classic 3D fixed everything!
Just thought I should let you know - in case anyone else gets stuck like me.