I own a Defy+ and I wanted to give it a try with kitkat. With stock ROM I had no stability problems, but when I load kitkat after a few minutes I get a message on screen , saying the modem has a problem and the phone wants to reboot, giving the choice immediatly, remind later or not reboot.
This happens with quarx ROM from 20.1.2014, but also with older versions. I even went back to CM10.1, but it has the same modem problem.
Someone suggested to change CPU and Voltage to fix a reboot problem, but it did not make a difference.
I found some instructions how to create logs when the problem happens, I followed this instructions
and cat /proc/kmsg into a file on sd card while the modem message appeared on screen.
The file is attached, I hope it is usefull. If something else is needed to track down the problem, pls let me know.