Engine RPM changes automatically

Hello All
i own a 1997 model opel astre car and from last few days
i am observing a new problem in it which is when the engine
is cold and i start the vehicle and accelerates the rpm suddenly
goes to some 3000 rpm and would remain thier constant without any control of accelerator paddel and i need to run the
vehicle like this for a s few km controlling it by cluthing and gears and things become normal after few KM
so please help

Thanks
Manish jain