Erick,

Flutter is the uncontrolled oscillation of the control surface. You may hear it as a "buzzing" sound at full throttle or in a dive. It can lead to a catasrophic failure of the surface or the plane. It's frequently caused by excessive airspeed and sloppy linkages and made worse by heavy surfaces.
Balancing your surfaces would certainly help. Just lightening them would make a difference. I would seal the gap between the stab and elevator, etc. Replace the pushrods with new stiffer units and eliminate the slop. use ball-links, carbon rods, stiffer horns, whatever you need to make the entire linkage bullet proof.

Chris