engine misfiring

my svx has been running fine until today when it started misfiring while i was driving on interstate at 65mph. when I came to a stop it sounded like it was idling on 5 cylinders and the car was shaking,...the outside temp was around 20 degrees. I did however buy gas at a different station lately but the tank is now 1/2 full. I've never had any previous engine related problems. does anyone know if the problem is more likely to be fuel related or ignition related. any suggestions would be appreciated.