I have an 02? Yamaha Grizly that on frosty mornings has continued accelerating three times. It only happens in the first 2 or 3 minutes of operation. Thankfully there is that nice red switch on the left bar. Each time i have opened it all up checking the free motion of all the components. I left the cover off the carb side to be able to see the butterfly position if it happend again. When it did, i killed it and watched the butterfly slowly return to idle. Could this bee ice forming under the slide and coming loose lodging in the butterfly?

 

Sounds like a bad deal. Beings it happens only on frosty mornings I think icing up probably is your problem. I guess I'd start with using gumout spray or similiar and clean all linkeage from thumb throttle all the way down. Then in the winter try some different gas and maybe put a little seafoam in it.