20:44:24.661: Video stopped, number of skipped frames due to encoding lag: 1352/1681 (80.4%)
Your encoder is overloaded.
Also, you shouldn't use CBR for recording, that's mostly for streaming. Use CQP and adjust to a level where quality is acceptable without causing stuttering or reduced framerate. However I wouldn't be surprised if the 760 weren't able to handle fast motion in 1080p60 without compromising quality a little, as in your logfile it is badly overloaded.
Also, don't record to mp4. Use flv or mkv and use the "remux recordings" entry in the file menu after stopping recording. Crashes, hangs or errors during recording will render an mp4 completely unrecoverable.
