vapor lock was exactly what I was going to say...when you take a break the radiating heat from the engine may be causing a vapor lock that goes away as the engine heat dissipates

 

that could be, we have rideing at 4500 FT or above and it has the stock carb jets.

 

Whoa, heating when driving in snow? Something is weird there. Had mine to 6,000 feet with stk jets. No heating problem, just sputters and looses power. And when I had the tracks on, I can't imagine working and engine harder. That is about 25 deg. F.

Something is causing it to lean out. Are you running without an airfilter or kn?

 

Another thought, check the sump in your airbox and see if there is some oil in the bottom. Let me know if so.

 

The Bike is Bone stock, 6 weeks old. I will check the sump and get back to you.

 

if you can get to it, pull the plug out and check it to see if its bone white or not. this would be a good way to tell if its running lean.

 

here is a little something i found over in the suzuki section that i slapped into my favorites....
this plug color chart should help telling how its running...

 

One other thought, if you were at or near 6,000 ft and it was a hot day, the 300 gets pretty finikey and sputters ( just like the manual said).

One day at 6200 ft and 60-70 degrees, it was running rough enough that I didn't want to shut it off (just in case). I took the lid off the airbox and it smoothed out. Got another 500 ft Alt before it got too rich to run. Shut it off and put the lid back on for the descent. Man, it was so rich it was hard to start but as I got down to 1900 ft, it had cleaned up and ran normal.

Maybe some of everyone's suggestions might give you a clue, and in no way am I hinting that you are clue less. lol & jk.

 

O.K. I'm gonna chime in because this reminds me of an old Triumph triple I used to own. It had the same problems you are having. Not to sound stupid, but the coil internal wiring separated when it got hot and it would quit. I dont even know if your ATV has a coil....or mine for that matter!!!duh. I should know but I dont so forgive me if I dont know what I'm talking about because, its justa guess and may apply to this problem you are having. Just my .02 cents....maybe .01 cent this time....;-)

 

Certainly a possibility, seen that problem many times.