I created a new project and switched the settings at startup and voila... the new project was playing back perfectly. I then closed that new project and re-opened my older project that was stuttering and now it was playing back nicely too.
I also did a quick export with matching sequence settings and it went about 3x speed. Conversion to H264 was pretty slow taking roughly 16 minutes for a 2 minute piece. 1920 x 1080 30p 5D/GoPro mixed footage.
Well, I hope this helps a little. I was searching for a while for this and got close but never saw anything about the playback engine. I did the rollback to 8.0.1 once last time but now I'll stick with the newest version as it seems to be working =)
Merry Christmas and Happy Holidays,