It really sounds like a bad plug. Even if you just changed it. Runnin on a small track or slow trail riding heats these 250s up and burns up the plugs quick. Also they tend to run on the rich side at low throttle settings so when you plug along slowly or if you are throttling and decelerating it builds up unburnt fuel in the case and cylinder and you flood it out while you are riding it. Both these conditions produce the slow chugging you are speaking of. A compression test couldn't hurt either. Do the spark plug first as it is easiest and cheapest. Good luck.

 

Sounds like its starving for fuel. When was the last time you changed the fuel filter?
Try that and put a new plug in.
My Honda Accord was doing something similar. @70mph it started to die. Slowed down to 60 and it was fine until I hit 70. As time went on I would have to have more gas in the tank. At first it started at 1/4 tank then 1/2 and so on....
Ended up replacing the screen on the bottom of the fuel pump in the tank.