Do you still have the same problem playing files from a direct connected USB Flash Drive ?
I didn't think to try this method. I can check.
Every time the system "buffers" I am seeing this in the debug log
2019-04-06 19:08:52.922 T:140081834751744 INFO: CVideoPlayerVideo - Stillframe detected, switching to forced 23.976024 fps
2019-04-06 19:08:52.922 T:140081834751744 DEBUG: CPtsTracker: pattern lost on diff 208541.666667, number of losses 4
2019-04-06 19:08:53.127 T:140081826359040 NOTICE: CVideoPlayerAudio::Process - stream stalled
2019-04-06 19:08:53.345 T:140082681988864 DEBUG: CVideoPlayer::SetCaching - caching state 1
2019-04-06 19:08:53.345 T:140081834751744 INFO: CVideoPlayerVideo - Stillframe left, switching to normal playback
2019-04-06 19:08:53.345 T:140082681988864 DEBUG: CDVDClock::SetSpeedAdjust - adjusted:0.000000
2019-04-06 19:08:53.345 T:140081826359040 DEBUG: CDVDAudio::Pause - pausing audio stream
2019-04-06 19:08:54.270 T:140081834751744 WARNING: CRenderManager::WaitForBuffer - timeout waiting for buffer
2019-04-06 19:09:04.383 T:140082681988864 WARNING: Previous line repeats 19 times.
2019-04-06 19:09:04.383 T:140082681988864 DEBUG: CVideoPlayer::SetCaching - caching state 2
2019-04-06 19:09:04.383 T:140082681988864 DEBUG: CDVDClock::SetSpeedAdjust - adjusted:0.000000
2019-04-06 19:09:04.383 T:140081826359040 DEBUG: CDVDAudio::Pause - pausing audio stream
2019-04-06 19:09:04.383 T:140082681988864 DEBUG: CVideoPlayer::SetCaching - caching state 3
2019-04-06 19:09:04.383 T:140082681988864 DEBUG: CDVDClock::SetSpeedAdjust - adjusted:0.000000
2019-04-06 19:09:04.383 T:140081826359040 DEBUG: CDVDAudio::Resume - resume audio stream
2019-04-06 19:09:04.384 T:140083894417152 DEBUG: ActiveAE - start sync of audio stream
2019-04-06 19:09:04.393 T:140082681988864 DEBUG: CVideoPlayer::SetCaching - caching state 0
2019-04-06 19:09:04.393 T:140082681988864 DEBUG: CDVDClock::SetSpeedAdjust - adjusted:0.000000
2019-04-06 19:09:04.524 T:140083894417152 DEBUG: ActiveAE::SyncStream - average error of -263.597715, start adjusting
2019-04-06 19:09:04.676 T:140083894417152 DEBUG: ActiveAE::SyncStream - average error -25.301511 below threshold of 30.000000
2019-04-06 19:09:05.774 T:140081826359040 DEBUG: CDVDClock::ErrorAdjust - CVideoPlayerAudio::OutputPacket - error:73131.695662, adjusted:73131.695662
2019-04-06 19:09:09.303 T:140081834751744 DEBUG: CPtsTracker: detected pattern of length 1: 41708.33, frameduration: 41708.333333
Is this 'StillFrame' a symptom or culprit?