It sounds like time for an ATV tune-up. On top of your regular tuneup, for this particular problem, I would recommend draining and safely disposing of the old gas, replace the spark plug and make sure your jets are properly tuned for your riding conditions.

Assuming that your ATV is not overheating it’s likely just not getting a strong enough combustion when fully warmed up. Cleaning the carb and changing the air filter can’t hurt at the same time but start with making sure the engine is getting clean gasoline and enough of it.