Considering the age and model of your car, it's probably time for a new ETA (electronic throttle actuator). My car has the same problem but usually lights up the Check Engine Light (which I have come to ignore for now). It will take a few more paychecks to replace the ETA and everything should be fine by then.
I forgot to add that it is possible that the bowden cable is worn out and the plastic sheathing has crumbled away.
Good luck.
