This problem has come on very gradually over the past few months, but, when accelerating or trying to maintain lower speeds of 30-40 mph, the engine will make a slight grinding/fluttering sound that corresponds with fluttering in the actual power to the ground/acceleration. This hasn't normally happened on every drive, but, is starting to become more frequent. At higher speeds it's not an issue and decelerating isn't an issue, either. The issues happen between 1500-2000 rpm and are gradually becoming more consistent and noticeable. I had a tune-up and ran some Lucas fuel treatment through without improvement, so far. Anybody have any ideas? Thank you, in advance, for taking the time to read this and for any feedback you might have.