Hi,
are these gas or electric planes? do you use balanced props?What do you change after an incident? Or do you take the gear and put it into the next(new) model? What about the battery - how old, which size? Do you use a switch: how old? Servos: which type do you use on the elevator? Where is it located? No slope in the servogear and the linkages? Loose covering? Loose hinges? broken cabels? Are the cables fixed in the plane? Corrosion due to salty air? May be some pictures would help...
Bernd