I bought a 1998 30 HP Yamaha 2-stroke. Rebuilt the carbs, installed a new impeller, changed gear oil, checked the compression (it is excellent on all 3 cyls.). I take it out for a spin and it does this thing in the video below; vibrating quite badly when I up the throttle (going slow is fine). I try shifting gears back and forth from neutral to forward and reverse, which seems fine while trying to figure out the problem. Five minutes later all of a sudden the problem stops and I'm flying on the water. After another five minutes I slow down and when I try to up the throttle again, the problem is back. I have tried adjusting the gear link but that didn't help.

I'm guessing it is one of these things: A worn prop seal which calls for a new prop (but as far as I know that doesn't cause vibration - just revs without speed), misadjusted gear link or gear cable from the controls, or worn gears (please, no, I've seen what those cogs cost to replace...)

I would love input on how to solve this - maybe someone here has experienced the same thing. Thanks in advance.

Video of the issue