No issues on my end. Been running great.
Have you frozen or removed any apps from stock since upgrading? You have to be very careful when doing something like that. I would instead recommend Greenify Module with the Xposed Framework. Instead of freezing or removing an app, this will hibernate them. That way, the app can still be used if you need it, but it isn't disabled which can help when it comes to a phone acting wonky like yours.
This does require rooting though, so if you don't wish to root, then I'm not sure what other options you have.
But, if you have frozen some apps, I would start re-enabling them (or enable all of them) and then start freezing them one-by-one to see which frozen app might be causing your issues.
If you removed some system apps (which means you're rooted), then you'll have to FXZ back to a stock image before you can start trying again.
Another user on another board was having issues with their phone acting up. He finally admitted to freezing/disabling some apps, but he couldn't understand why it worked under JB and not KK. Issue is, KK might have some apps that depend on the frozen apps that wasn't present before under JB.
Otherwise, I'm not sure what else we can say. Are you on stock as stock can be? If you haven't made any modifications to the system (root, freezing/disabling, removing bloatware), then I'm at a loss because my phone has been working just great.