I second the air leaks. What worked for me on one bike that cured what I will call a "sputter" where the bike scrubed some speed then immediately recovered when cruising at 1/4 to 3/8 throttle:

Using Permatex Copper Spray-A-Gasket sealant (the high-temp stuff in the spray can). Even though I used a brand new Yamaha gasket and cleaned the cylinder and head mating surfaces, I didn't try to make sure those surfaces were entirely flat. The sealant took care of the irregularities.

One piece of advice I've heard is that a brand new copper gasket may not necessarily need sealant, but reusing a copper gasket could benefit with an appropriate sealant.

All that, plus setting the float height so the bike was no longer running on the rich side of things, and setting the needle clip back to second from the top (the stock position), and I thought I had an entirely different bike!

Hiya,not much help but back in the 70s I had two dt400s twin shock and mono,both “lunged”...annoying but I got used to it,over 40 years later I have just bought a 1975 dt400b,it’s lungeing...just how I remember it!