Hello boys and girls

I got a problem with my cforce 850xc if any one have the same isue or can help me out.
So it only happens when the engine is hot (not overheating) just at temperature, when i accelerate its like the belt slips or the engine get stuck when im between 40 km/h and 50 km/h. I already go 3 times to the dealer and he says its fixed but not, i change the spark plugs, the atv is in high gear. He is 4 month old and 1250 km on the counter. I made a video on youtube showing the problem. https://youtu.be/pjsT4hcNqQM
If any one can help im lost now 😞
Mush appreciate

Just guessing, but sounds to me like misfire.
Have you tried the tank cap trick? Leave the cap slightly open for a test. If problem goes away, look up for the evap canister recall, or just blockage on the tank breather.