After some research I have concluded that this issue predominately occurs with the latest versions of mpv, vapoursynth, vapoursynth-extra-plugins and libass5 from the djcj repo. I have been able to replicate with older versions too, even though it was far less prevalent there. Meaning, that SVP is definitely involved.
Not exactly sure what the problem is, even after consulting wmf at github, so I hope you might find an answer.
Using Lubuntu 14.04 and the latest version of SVP4Linux.
Instead of the output framerate being at an expected 59.9FPS it is sometimes lower or higher.l, while repeatedly playing the same file.
Here some log outputs of the same Youtube Video being played multiple times in a row.
"22:06:09.876 Playback [2e751307]: enabled while video is playing
22:06:09.896 Playback [2e751307]: playing at 59.9505 [59.9505 *1/1]
22:06:23.359 Playback [2e751307]: enabled while video is playing
22:06:23.375 Playback [2e751307]: playing at 52.7546 [52.7546 *1/1]
"22:07:09.876 Playback [2e751307]: enabled while video is playing
22:07:09.896 Playback [2e751307]: playing at 89.9205 [89.9205 *1/1]
"22:08:05.636 Playback [2e751307]: enabled while video is playing
22:08:05.656 Playback [2e751307]: playing at 74.3105 [74.3105 *1/1]
Any ideas?
At first I thought it might be composite, as I use KDE with vsync, but it still occurred after turning it off.
It must either be mpv, SVP or both.