yamaha xt 250 1991

hi i hope someone can help me with this problem i have a yamaha xt 250 1991 model and its starts and runs well when its cold but as soon as you take it for a ride and the engine warms up the idle jumps up to about 3000+ rpm and if you flick the accelerator it will go back down but with a chance of stalling. also when it is idling if you accelerate it has a huge flat spot and sometimes will stall any advice would be greatly appreciated many people we have asked seem totally confused by this

thanks James