It sounds like you've got the same problem as some others out there (including me). I'm just getting into researching this problem for my truck. I found this thread...
http://forum.gopowerhungry.com/gryph...puttering.html. It described my problem pretty accurately. I still haven't found the cure, but I'm sure there's one out there...
There are plenty more threads on the forum about it, search the word "sputtering" and you'll find a lot more info about it...