Samsung Galaxy Note 3 — Keyboard Getting Weird

I have had to replace my T mobile Note 3 four times in last two months. Fortunately it has been warranty, but still frustrating. Could be my own fault, but entirely different issues with each one and I know I'm getting refurbished phones. This one quite often will add nonsense letters in addition to or instead of what I type. It can take three or four times as long as it should to respond to an important text. Doesn't happen every time and T mobile says they don't have any idea what I'm talking about. I've even gone so far as a hard reset with no luck. Tried a different keyboard too, but it was too difficult for my limited understanding of technology and associated terminology. Can anyone offer suggestions? Thanks.