I thought I'd contribute to this thread. I just got my new m8 yesterday, and I have the same issues with thre twitchyness. Keyboard, back button, ect. Also when browsing the web, I notice that the ads seem to open themselves up on there own, they either bring me to a new page or open the play store. It's rather infuriating. I do not have an SD card installed, I use the stock charging cable and wall adapter.
I see from your profile that you're on Verizon, is that still true?
If so, both you and the OP are.
So, I guess I wonder if it's either a weird batch that went out to Verizon (so the OP's swap to another didn't help) or something in the overall Verizon software configuration (which, for no rational reason beyond a hunch is what I really suspect now).
Try the app I mentioned earlier - if it doesn't help or you don't like it, you can uninstall it, it's harmless. If the button problems go away, it's software.
By the same logic, try the free version of the Nova launcher - same deal as above - either or both would be fair enough software changes to tell if it's just software.
Meanwhile, if you could both share your version numbers that you're running, that might help give some clues also.
If it's just software, you may see it disappear on an update, or you could root and swap out your system software for something with less carrier junk.
Btw - try something besides the stock browser. I like Dolphin, for example. The browser behavior you're getting would make me crazy.
