I have playback issues on my ROKU Ultra (4660RW, Plex 5.1.1). I'll call it "choppy frames" ala silent movie era!!!! Consistent playback but looks a little like the old "flip card movies" you'd make as a child.
On this setup I have a Yamaha RX-A1020, a PS3 FAT, a ROKU Ultra, and a Vizio
Playback of the same content is fine like live TV on the PS3 (Plex 3.1.0, PS3 01.41).
Video playback of the same content is fine using the Vizio app (Plex for Vizio 3.13.13, Vizio Presto 0) as well. Unfortunately Vizio hasn't updated the client on this TV in awhile and it has the "closed caption always on" bug that Plex supposedly fixed for Vizio.
I enabled remote logging on my ROKU and played about 5 minutes of the show from my server (Last Man Standing) just prior to hitting Post Discussion!