Looked at the uTube clip a number of times, and think I see a little bit of movement at about the time she is losing bite -- though maybe imagining it after viewing too often.
Is that motor flexing on the transom? I don't see a thrust rod in place, so assume the motor isn't locked down. Also don't know how solid the transom is.
Additionally, I'm hearing a little bit of roughness (chugging) at start up, although It seems to even out some -- not sure if that's symptomatic.