At the beginning of the flight, the model should be nose heavy respect to the ideal recommended CG position, due to the fuel weight.

Therefore, it seems to me that the problem may be in the incidence angle of the wing.

I believe so, because the tendency to tuck down that has to be compensated with strong elevator may indicate the wing positioned by the stab slightly negative and close to zero AOA.

I suggest trying increasing the AOA a little bit or verifying the current decalage with an incidence meter.