Try a range test. Plug the rcv into a servo and get somone to walk away with the transmitter until you find a distance at which they can no longer reliably move the servo. If that distance is longer than the distance over which you need to control the robot, than your aerial (in fact your whole radio control link) is OK. If not, add wire up to a total length of 90 cm. At the same time, watch the servo for the twitching you refer to above. If it does not twitch, then the cause of that is somewhere else in your system.