I have a 2000 (or it might be 2001, whichever is the first model year) Ford Focus. There are about 85,000 miles on it. In the last week and a half it has been shuddering. It doesn't last too long and somedays it doesn't do it at all. A friend said that it might be the fuel injection system and suggested using a mid or premium gas and using a fuel injection cleaner or something. Could it be this or something else?