Flutter is a very complex subject. Everything (yes EVERYTHING) will flutter if given the proper input (energy at it's resonant frequency). The only thing that will not flutter is something that is infinitely stiff and has no mass an impossibility. Our choice is to change the resonant frequency of whatever is fluttering so that the input stimulus is not at the resonant frequency. How do you do that? Two ways, change the stiffness or change the mass or some of each. The easiest way in most cases is to increase the stiffness of the structure that is vibrating (fluttering). The solution in most cases is to move the resonant frequency (by increasing the stiffness with sheeting or more rigid components). Perhaps, since you say it is slightly nose heavy now, is to sheet the surfaces with 1/32 or 1/16 sheet balsa to stiffen it up. This will not add much weight and will increase the stiffness a great deal.