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That's rather odd, have you tried
settings > apps > all > camera > clear data
also, are you saving the videos on the internal storage or SD card?
have you encrypted the phone?
Are you able to test and let us know if the same issues occurs with removable data(SD card)?
I'm facing the same problem, more or less on my Z5.
Sometimes video preview (while I'm recording video) is laggy. Some of these lags also found on he recorded video.
Happens only @30fps , not @60 (till now)
No matter if I use internal memory or sd (my sd card is class10 uhs3).