Fluttering is kind of confusing. Are you "lugging" the engine? Try downshifting without increasing your speed. That will raise the engine revs. The programming of some of these cars is designed to upshift early in an attempt to increase fuel mileage by decreasing revs, but at times that can cause lugging, which eventually leads to nasty things like carbon buildup or ring failure. See if a lower gear smooths things out.
