Maurizio-
If you still have a stock '68T kettering ignition you may be seeing this cold misfire due to the combination of a weak spark and a cold mixture of air and fuel that isn't vaporized.
You may be able to correct the misfire by replacing the conventional kettering ignition system with a Capacitive Discharge Ignition system from a '69E or S (or other early car CDI system if you wanted to keep the car "correct"). Another option is an aftermarket CDI system such as MSD or Crane.
http://www.pelicanparts.com/catalog/shopcart/911E/POR_911E_ELignt_pg6.htm#item31
Good Luck,
Jim