Here is the possible prob. On my 92 xcr the same thing was happening, at first i thought it was my cdi box getting heated up, but it turns out that some levers in my carbs were worn out. When the sled warmed up, the (levers as i call them) that the throttle cable pulled up on would slip
Remember: DON"T HESITATE.........ACCELERATE!!!!
Remember: DON"T HESITATE.........ACCELERATE!!!!