Have you tried programming the cortex through the USB cable? I have cortex in my Funtana 30cc and its alsolutely locked-in In both precision and high alpha manuovers. I suggest you bump the gains on the rate mode till you find oscillations on any of the surfaces and then reduce the gains on the respective Surface ( the one that oscillated) by 1-2 clicks on the programming interface. For pattern you could also reduce the stick priority and increase the latching priority. Usually the ailerons would start to oscillate first when gains are bumped.
I can do a slow roll on the rate mode with out giving rudder and elevator on the cortex. The way the algorithm of the cortex is written, it dosnt need the heading hold mode. So basically you can program the unit to have rate modes only and you could try different gains, latching / stick prioritys. Try this before giving up on cortex and I am sure it would work for you.
good luck !