The others' ideas were good, but let me offer an altogether different possibility, because your descriptions strikes a chord...

I really sounds a lot like insufficient fuel delivery, in other words a bad fuel pump, but since it's only occasional...

By any chance, did the times when the sputtering occurred happen to be on a hill (especially uphill)?

I'm thinking that you may have the dreaded "decomposing fuel hose" -- the short (~2 inch) one that's between the transfer (or in-tank or pre-) pump and the metal fuel line attached to the fuel level sender inside the fuel tank. Notoriously, this short hose splits and, when the hose is above the fuel level in the tank, allows bubbles instead of liquid fuel go to the main fuel pump. This uncovering of the hose occurs when either the fuel level is below 1/3, or when you go uphill (when below about 2/3 of a tank depending on the steepness of the hill).

Or maybe you have a lot of crud in the tank that's clogging the "sock" (the strainer that's upstream of the transfer pump).