iPhone 5s - keeps turning off and can't turn back on
After ios11 update my iphone 5s doesn’t stay on, not sure if that's just a coincidence. The phone works quite happily if I can get it to turn on which is often a challenge (I’ve tried pressing the home button while holding power button which usually yields better results but doesn’t work every time). Once it’s on I can load apps, surf web, check emails and everything functions normally, I’ve done this happily for anything between 2 mins and an hour but the problem appears to happen when it is in standby. After a short period of time I find it has switched off / black screen and it takes a lot of effort to get it back on, if I can get it on at all.
If I plug it into a pc when it is off / showing a black screen itunes recognises it’s in recovery mode and I can do a restore. I tried rolling it back a version but now os 10.x is unsigned I could only go to 11. I’m guessing its reverting to dfu mode if itunes can see it but I'm not sure as it won't come back on easily. I’ve replaced the battery and this charges up ok. Tried all the standard stuff, hard reset, etc. although most times this reset results in me struggling to turn it on again.
I’ve now bought an se. I was able to install all my se settings/apps to the 5s without problem (phone to phone transfer) and it updated to ios 11.2.5. Again, I used it for a while before putting it on standby. I left it plugged in this time, popped to the loo but the same result when I returned a couple mins later.
I’ve read about replacing the U2 IC but I’m not 100% this issue is completely consistent with the issue. Any thoughts on how I might resolve this are appreciated?