Looks like several people have the same problem here. The mech told me Friday that mine is an intake leak. Makes sense, my Harley did the same thing. It had 2 O ronigs for intake seals. Sometimes it would run for days just perfectly, then suddenly it would spit & sputter. Changed the intake seals and all is well. hopefully they will find the leak in mine.
How they found it: When the car is not running well, spray some flammable (starting fluid) around the base of the fuel injection system. if it speeds up, your intake is leaking.