How to get stable rotation values?

[.:HyPeX:.]

I'm working out an IA which will control a helicopter, and mostly will just keep its rotation within the angles specified.

So far said IA does fine when rotation is applied and height, keeping the helicopter suprisingly quiet. However, whenever i add any angle to the X or Y axis.. it works fine for the first.. 3 seconds? then the rotation axis of Y kicks in and it goes to whatever number and the maths no longer check out. (When the Y axis is flipped the X axis is affected aswell) so i'd like to know how can i properly keep them stable in a 0-360 basis.. and not have them flipping arround whenever another axis alternates (Why is this even a thing?!)

Greetz.