all of a sudden my blazer started running poorly. at idle it feels like it is running on about 7 cylinders(shaky really rough idle). When I stomp the gas, though, all power is there. While cruising though, the truck will sort of sputter as if it were getting no fuel for a quick second. it will do this sputter two or three times and then is fine. If i use very little gas i dont have this problem. What could this be. my wild guess might be fuel pump or fuel filter but this just began half way on my way to work today.