Kitty Cat Snowmobiles

I have been having a problem with my son's kitty cat and can't seem to figure it out. If it is about 10 degrees or so below freezing it runs great and he can go for hours. However if it is just around freezing or slighty above, it will work for 10-15 min and if he stops at all, it won't go again. It will bog and eventually die. The engine is really hot. If I leave it for a minute or so, start it up, lift the track and "clear it out", it will work great as long as he doesn't stop again. Is this a problem that others have with these? I would think they should run fine in the heat as there are wheel kits for them but I am stumped.
It's a 92, good compression, new plug,new jets in the carb.... One more question, how easy should the track spin? If I try to spin it by hand, I have to really pull on it to get it to move. Seems pretty stiff to me compared to a larger sled.
Thanks
Les

very well possible vapor lock
make sure your fuel system
is well ventilated happened to
my sons today loosened the cap up
and ran 3 1/2 hours non stop and
trouble free!
KITTY CATS ROCK!

I thought of that as well. The cap is pretty well vented. It has a good size hole in the top that I can blow through with no problem. I did try loosening the cap off and it didn't seem to make a difference. The fuel line is barely warm as is the carb. The cylinder head on the other hand is really hot. Hot enough to melt the rubber on the plug cap to the plug.
I took the drive shaft out to check the bearings and I think they might have been seizing up. I replaced them and the track seems to spin more freely than before. Maybe it was just working so hard it was overheating?
Haven't had a chance to try it out yet.