Hi Pierre. Tried all that. Still getting the same problem.
I uninstalled all the old versions of Twixtor as well- thought maybe they were causing a conflict but with no luck.
Have now uninstalled Twixtor 5 and everything is working OK again.
Even when I had Twixtor sitting outside the Adobe folder I was still having the problem – had to completly remove it from my system before playback worked OK.
The video footage I am using is from two Canon DSLRs (Canon 5 D MkII and the 1D MkIV)in the 1080 25p mode (I am in Australia). Even when I render the footage I still get the playback problem and I am not even using Twixtor.
I am not a novice and even had a very experienced Video IT guy have a look as well but we are befuddled! My software is a properly licenced upgrade from Twixtor 4.5 which is an upgrade from version 3.