If the sudden pitch down is extreme, then there is a problem with the Safe select function in the receiver. I have seen it happen several times. I lost an Extra 300 due to a sudden pitch down to vertical with no elevator control when switched to SAFE mode. However if the pitch change is moderate but controllable, then consider this scenario: In the "beginner" mode, the gyros built into the receiver limits pitch and bank angles and adjust the pitch angle to automatically climb at full throttle, cruise at half throttle and descend (glide) at low throttle. If the elevator pitch trim setting is not where it needs to be, say too much nose down, the "beginner" mode will apply what ever "up" elevator is needed to achieve a pitch attitude that is appropriate to the throttle setting for climb, cruise or glide. Switching to "intermediate" mode turns off this automatic pitch adjustment function. This results in the airplane reacting to the elevator setting at "neutral". Get an experienced fly to properly trim out the airplane using the "advanced" SAFE setting and then try the 'beginner" and "intermediate" modes to see if that cures your pitch down problem.