To help eliminate one potential source of trouble discussed above, this isn't about CG if the plane is controllable when it leaves the ground. If it's about CG at all (and I have serious doubts about that), it's about the CG moving far enough to make the plane uncontrollable while in flight. Like a battery free to move the entire length of the fuse or something...

If the plane left the ground with a fixed CG that could potentially make the plane that unstable, that fact would be apparent from the time it leaves the ground. It would not happen out of nowhere, mid flight, or after 40 or 50 flights.

I'm left wondering about combatpigg's line of thought. Has the elevator been set up in a manner making very sensitive elevator inputs required for normal flight?

More info on the planes themselves (e.g.ARF or stick built), type of power, etc. might generate some useful ideas.