I don't want bring up an old topic but I'm encountering a similar problem. Slightly different than other posters but similar enough to not start a new thread, I think. I thought I should ask for advice first before going to the store.
I have been using the fast charger for over a month now. Nothing out of the ordinary. I've notice my phone feeling a little overheat once or twice in the past but that it. Nothing noticeable. My phone is up to date, 5.1.1 as of about a week ago. About 3 days ago, I noticed that the sensor was a bit off. If I tilt the phone 15ish degree to a side, it will switch the landscape mode to that side. A little too sensitive but I didn't pay much attention to it until it worsened today. It wouldn't go back to portrait mode. I could slwo tilt it the other way, and it will go landscape mode the opposite direction, skipping the portrait mode all together. The only way for it to go back to portrait mode is to close whatever app I was using, put the phone down on a flat surface. But if I pick up the phone again and tilt it 15 degrees to either side (easy to do by accident), the phone will go into landscape mode again.
I downloaded cpu-z. Not entirely sure how to read the numbers but as far as I can tell, all numbers changes when I move the phone. Nothing is "stuck." I also used Check My Android and all numbers and lines change when I move the phone.
I have tried restarting the phone. I also clear the cache in the startup screen (press and hold the up volume, power, and home button after I have power off the phone). I haven't done a factory reset my phone, which I hope I won't have to. But probably will soon.