Can anyone diagnose the following outboard problem?

Nissan 6hp kicker motor. Very calm water. 3/4 throttle. Hand temporarily off the tiller handle, but boat still moving well on straight course.

Suddenly motor reves very high and turns abruptly to the side. No evidence of any debris in the water, other than a floating intact supermarket plastic bag.

Does anyone have any suggestions?

Thanks.

Sorry to read this late but it might have been that bag or a piece of kelp in the prop. You might consider tightening the steering tension so it's not deflected as easily because someone could get thrown overboard with a sudden swerve like that. My engine has done exactly what yours did, sometimes turning, sometimes kicking upwards. I never take my hand off the tiller at speed, only at idle due to safety concerns. Also, do you use the kill switch lanyard attaching it to your wrist or a belt loop? You should because if you go over and it's in gear and moving you may be cut by the prop or left stranded watching the boat keep going.

Thanks, Fongster; this is good advice. Actually, this incident happened to my son, not me. This is his first outboard. I will pass your message on to him; it will contribute to his safety and to my peace of mind.