Happens only in high gear.
Does not happen in low gear.
I humbly ask to please tell me the cause and how to fix my problem.
Sounds like its not getting enough fuel. Possible plugged screen in fuel tank?
Or a plugged gas tank cap vent?
I am not buying the fuel issue IF and ONLY IF this issue is limited to high gear with smooth running in low gear. The reason is that fuel flow needs are higher with higher RPM and you are having the issue when you LOWER the RPM. I think you have a spark plug gapped too wide or a weak coil that is missfiring under load which is what happens when you shift to high and place that heavy load on the motor. Check your plug gap first and set it to 1/32" (.032) and then test your coils with an external gap connected to the end of the spark plug wire with the plug disconnected and verify they each and every coil can produce a spark that will jump 1/4" with no plug connected. DO NOT USE the spark plug laying on the head as the test gap since that is a false test. You can and should do this at idle if you are running on magneto but at higher RPM if you are using battery only. The reason for that is that you want to test at a point when the power to the coil is weakest. I honestly suspect it is a coil.