1980 DT? Electronic ignition? Because old 2 strokes with points could get out of time on a gear change. My IT-250F had solid ignition by '79 but the older DTs (pre-76) were finicky.
Dollars to donuts it's fuel starvation. If the tank, filter, petcock and fuel line are solid, my money is on a carburetor reassembly error. Your bike has a Mikuni carburetor. Mikuni carbs are rock solid and dead simple - unless it's removed and reassembled wrong. They still appear to work, have limited function, but the flow in the bowl is compromised-
Don't swap it out, it's likely fine... but check a few things.
There's a rod/needle in the center of the throttle slide. The top of it goes up through the throttle and attaches with a tiny circlip to the slide. Cant recall the attachment to the return spring, screw cap / snout which then somehow hooks to the throttle cable. But it's in there.
The bottom of the rod sets down into the main jet/valve in the bowl. I've seen more than once, after the carb's been removed, and the screw cap and throttle left hanging on the cable, the circlip holding the slide in position on the rod will end up on on the wrong side of the throttle slide. It gets loose as the fuel really starts flowing, sets back into the main jet, and cuts off the fuel into the throttle.
I also recall some interplay between that assembly and the float lever and float valve. I can't explain why, but the fuel demand isn't linear in the higher gears, like when you're accelerating in 2nd, and the motor gets starved for fuel when it was fine in 1st.
It's been 30 years, pardon my vague recollections, but I maintained a stable of 1970-1984 japanese 2 strokes. They can run, and run well, with cooked rings or loose and sloppy jugs. If you're not convinced, screw in a compression gauge before pulling the head. No need to open that can of worms if you dont need to.