The flutter was either a building error or pilot error. Not bashing you because it's a common problem among new builders. I've lost two planes to flutter - both were my fault.

You have to keep that hinge line tight and preferably sealed. Use at least 3 hinges on any control surface. For example, if there are two elevator halves connected by wood or wire, then three hinges on each half minimum.

No slop in the pushrod. If the servo has a lot of slop in it then don't use it. Get a new servo, new gears or whatever.

Keep the control surface as light as possible. Square off the trailing edge of the control surface so air breaks cleanly from it. A rounded trailing edge will contribute to the potential for flutter.

Lastly, learn to use the throttle. The new 3D models are designed to be very light and over powered. That power is there to pull vertical - not to be used for speed. If you were to hit the throttle on some of those planes in level flight or a shallow dive you could probably kiss the plane goodbye.