I get into a problem when descending and climbing with auto pilot on. I cannot work out the correct procedure for the controlled climb and descent procedure, sometimes I get it right it will climb and descend at the rate of climb or descent I choose or it seems to have a mind of its home, now I suspect there is a correct procedure to get the aircraft to climb as I want it to. I have sometimes not been able to to stop a rapid descent ending with a crash. Turning the auto pilot off and pulling back on the joy stick sometimes is no help at all. What am I doing wrong ?