Sounds as if you have a throttle control problem.
If your car has a throttle cable from the accelerator pedal to the control under the bonnet it sounds as if the cable is sticking. If so it may be possible to free it off by lubricating it thoroughly, otherwise a new cable could be the answer.
If your car has the later "fly by wire" electronic control at the accelerator pedal, there could be a problem with that, and the plug-in unit may need replacing with a new one.
ChicksFan, April 2010