[Q] keyboard return always to samsung as default

i have Samsung note (N7000) with 4.1.2
i am going to "setting", "language and input" and choose "swiftkey", as default.
after about 10 minutes the default is changing again to samsung keyboard.and so all the time.
even i freeze "samsung keyboard" with "Titanium", the default is still changing to "samsung keyboard",but it is not opening any keyboard!
what can be the reason?
thank you
jacob