Accelerator pedal sensor

Hello ! I think I have problems with the accelerator pedal, when I press pedal I am unable to keep a constant rpm (ex 2000rpm)the rpm indicator on the clock goes up to the maximum even when i try to hold pedal and keep a constant rpm. Is this normal? The engine is running fine, I've got new throttle body , a clean air mass meter, everything is in a good condition.when I drive the car everything is ok, only when I'm stoped with the engine at idle I've got this strange symptoms . My car is octavia 1.6 l petrol 2001.could anybody give an advice? Also I don't have eny error codes or lights on the dash. Thanks , John