Hal 9000 said:

I have played v1 on my Roku 2 XS from start to finish when they were first released, with no issues. I just tried playing v3 and found... stuttering video, as if it just can't quite keep up. I rendered three test clips of varying bit rates, and all of them stutter regardless.

So, I am going to (try to) reacquire V1 or V2 from myspleen and test it. It's possible that a software update on my Roku could explain this, rather than any changes with my encoding settings. The most helpful thing right now would be to have people experiencing stuttering problems download v1 or v2 of any of the films and test them as well. If they have the same problem, I'll have to conclude that it is unavoidable and give up. If the older versions work fine, I'll keep investigating. (Though even lower bitrates than what I used before cause my Roku to stutter!)

Thanks for checking this out. It's a head-scratcher isn't it, I've looked at all the obvious video codec information and I can't see what would cause this stuttering. I'll have a look for an earlier version and test that out.