yamaha grizzly that just wont run right

Hi everyone, i am new to this site. i have a problem with my grizzly and am hoping that someone out there can help me fix it. I bought a 98 600 grizzly back in march and still havent gotten it running right. ive put 1000 bucks into it in parts alone, new clutch, coil, carb kit, etc, etc... the problem i am continuously having is the bike backfires and quits after it has heatened up and been riden for a few minutes.... after waiting a few minutes it will start up and run for a few more minutes before quiting again.... ive redone the carb, flushed out all the lines, changed coils, new battery, and so on. i dont think the cdi is bad, but i am running out of parts to change. does anyone know what could be the problem. Thanks and i appreciate any help you can give me. Wade