Even at proper dosages some cold medicines can make people excitable. I simply can not take anything with sudafed in it because they make me VERY jittery- even when I only take half a dose.
Cutting the dose in half would be a great idea, but if you are still jittery then I would TRY to avoid taking it at all. Humidifiers and Vicks VapoRub on the soles of your feet at bedtime (seriously!) can work miracles.