Constant gaining altitude at idle position of Logitech extreme 3d pro problem...

[JG26_EZ]

If you only set one button for trim, it must be a slider (so that you can move
your trim, up or down). Otherwise, you need to look for elevator up, AND
elevator down (two different buttons).

Remember that trim is a very fine adjustment which you're probably not going
to notice if you hit just once either. Try getting yourself flying as level as
possible, and then start messing with your trim to see what happens.