First, when everything is turned on, make sure all control surfaces are in line! Meaning, the elevator from a side view is dead on straight to the rest of the horizontal stabilizer, the rudder is not a bit left/right, and the ailerons are not a bit up or down. Balance the plane as in the manual. Make marks on the wings for the balance point, and put your fingers on them. The plane should stay level, or just slightly want to drop it's nose. Dropping the tail is very bad.
Are you taking off from the ground, or hand launching it? Hand launches should be smooth and level. Usually without much force behind them, if the motor is already running. It's easy to launch wrong in the beginning, and then getting the behavior you described. Find a field with tall grass, and with the motor off practice throwing it, so that you get a level glide into the grass.