I own a 04 polaris sportsman 400. When it's hot and I run it up hills it cuts out almost like I'm in reverse without the override switch pressed. I've also noticed it does the same thing when trying to accelerate quickly on level ground. I saw a post below that a fuel system cleaner/dryer may be needed. I'll try that tonight when I get home, but any other help would be appreciated. It seemed to happen right after a put new tires on the machine, maybe a carburator adjustment. Anyone have any ideas.
Thanks

 

Sea foam for a start to get out an water or fix the bad gas if it has some, is your fan comming on at all? Check your spark plug, wire, and coil for damage.