I've got an 07 Polaris Sportsman 500 EFI, with 80 hours and 400 miles on it.

Recently it has been having a bad miss at low speeds that clears up when i give it a bunch of gas and romp on it. It does not do this all the time though, it only does it when it is hot, when i am driving it around while it is warming up it runs like a top... no problems... but if i turn it off or drive it till it decides to screw me, it will act up.

I have replaced the spark plug
air filter, but not pre filter
tested the Stator, found to be in working order, replaced the trigger anyway.

it has been getting worse and worse and now after driving for 5 minutes and i turn it off, and turn it back on it sputters real bad and dies on me. It was doing it with 87 octane so i switched to 89, did not help at all.

what could it be

 

Try the Polaris ATV forums site.

 

adjust the valves then test compression

 

Well, time to get more into the engine.

Valve adjustments?
Is the carb clean?
Proper jets?
Piston rings?

You haven't flooded the engine or had any water in it in the past have you?

 

It's could be the fuel pump or fuel pump regulator. Put a fuel pressure test gauge on the fuel rail and monitor it when it starts acting up. If it pushes a solid 39 psi, your pump is okay. If the pressure drops, it's the pump.