Well I seem to be answering my own questions. Turns out my sim card is bad. I tried my wife's and the phone went to LTE and stayed. I put mine in her phone and it stayed 3g. So hope this helps anyone else that would experience this. Glad its not the phone.
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I'm actually having the same thing... keeps switching to 3G randomly, after a reboot it'll go to 4G LTE and usually stay, but often if I make a call, or do something it'll switch back to 3G for some time, then maybe go back to 4G... stock KK rooted here too
Did the SIM card fix it for you permanently? I may try getting a new one if so.