Hi,
It is quite likely that whatever causes flutter is visiting your model now, and that flutter itself could take place. Keep your airspeed down, check for slack in the model's elevator servo, pushrods, horns, etc. Did you 'Y' your elevator pushrods? If so, you may be getting flexing at the 'Y'. The model gods have given you a warning-find the problem(s) with your elevator system!! Good Luck!!
hookemut