YZ125 Sputters/Chops at high RPM (Video Included)

I'd start by taking the carb apart and double checking everything. And also the seal on the reed block. I'd clean that up, new gasket and thin coat of moto-seal, just to make sure there isn't an air leak there.

Could be power valves also. This might be the well-known press-pin coming out of the power valve issue the YZ125 has.

Air leak? spray some starting fluid around areas where there could be an air leak and see if the bike revs more. Sounds lean? Most likely....check the jets and make sure they aren't clogged with a piece of sand or dirt.

Check your reeds too, make sure they aren’t chipped

My CR250 was doing this, out of the blue, last year. Turned out to be gunked up ethanol crap in the carb from when I let it sit too long with fuel in it. I'd very carefully clean the carb as suggested above. When is the last time you repacked your silencer? If it's been a long time, you might try that too. You'd be surprised at what a difference it makes in how your 2 stroke runs.

+1 on cleaning the carb out. I had my shifter kart do that to me before where it would cut out like that and kill the starts, but be fine once I got going. Took the bowl off and there was gelled fuel from the fuel pump from sitting that would block the main jet only under full engine load like a start, but not once I was moving and at wide open throttle.