Flutter is caused by too much flexing in the control surface linkage.

I switched all my planes over to PULL _ PULL Dacron fishing line cables. What a huge difference. I snug the lines a little. Put a TINY drop of CA in 1 thread hole at each end of the control arms. Want to change holes ? Snip. It is SOO light compared metal or plastic. PLUS you can paint the exposed lines.

I am tired of tooting the horn.