I had a bit of boot loop, sometimes it would restart itself for no apparent reason. There was an issue with the on/off button. I took the whole phone apart and

I had a bit of boot loop, sometimes it would restart itself for no apparent reason. There was an issue with the on/off button. I took the whole phone apart and cleaned it out with a big brush. It was full of pocket lint, dust, etc. Even removed the on/off button (which is a fiddly little thing). It had a bunch of gunk on it (probably from my hands, you know, the stuff you'll find on your mouse after a while). Cleaned it out and put it back together. No issues since.