Old 2 stroke performance issue

[Tenntrapper]

I have a 1970 Johnson 20hp that has a problem that I can't figure out. It starts and runs great, however at about half throttle to WOT, it occasionally feels like I'm hitting something in the water. It happens maybe every 15 to 20 seconds...kind of jolts the whole boat. I've watched the motor, and it definitely shakes it. I'm thinking maybe a misfire, but not sure. Has anyone else had anything like this happen, and can point me in the right direction? It is a pin drive motor, and I've checked that for slippage...it's all good. Below about half throttle, it doesn't do it. Thanks for any help.

 

[Big Ben]

Fresh gas and oil mix. Use pure gas if you can find it. My old Evinrude would do the same thing and I found water in the tank. Plastic tank should not sweat but somehow it got water in it from the gas station I think. Mixed up a new batch from another gas station and it runs fine now. A guy that works on old motors told me to add a extra 1/4 oz per gallon in the mix will not hurt anything but help those old outboards out.

 

[Tenntrapper]

**UPDATE** Since my original post, I have fixed the problem. Even though the shift lever was never moving, apparently there was misadjustment in the shift linkage. For a split second, it would allow the ring and pinion to jump a tooth. My nephew pointed me in the right direction to adjust said linkage. All is good now. Thanks for the help.