Searching No Signal then turns off and on repeatedly. How to fix?
I bought this phone on ebay as faulty and locked to Vodophone. The fault described on the auction was that after replacing the screen it was working fine but then began to power off after an hour or so. All the buttons seem to work fine although the display is not quite as bright as my other fully functioning 3GS. I have had it officially unlocked for free by Vodaphone and I am now using a new Giffgaff sim on the O2 network in the UK. At first I tried a new Vodophone Sim. the problem was the same with both sims.
The phone boots up as normal and usually has a decent mobile and wifi signal to start with. If the phone is left alone after turning on it seems that the signal will stay for hours. If used, however, it will at some point turn off and then go into a constant reboot cycle sometimes getting stuck on the Apple logo for ages. A hard reset holding the power and home buttons for 8 or so seconds will kick it out of the loop, but if you turn it back on after 10/20/30 seconds or so or even after a few minutes it will go straight back into the loop or boot up with no signal and then go back into the reboot loop.
I have found that if I can get it to reboot to no signal and perform a 'power off' slide immediately and then leave it for an hour or more then it will boot up with a signal again and stay on with a signal as long as it is not used.
Restores in iTunes work but don't seem to make any difference. I have tried a dfu mode restore too but that doesn't make any difference either.
The problem isn't caused by apps because it had none on there originally when I set it up as a new phone. I have tried several times to get apps downloaded but it would turn off halfway through the down load. I did manage to get a couple of apps installed (Facebook and Ebay) last night and the problem is the same.
It has turned off after sending a text message and today I made a 5 minute call without any problem except that immediately after the call ended it showed 'No Signal' again so I turned it off with a 'power off' slide and when I turned it bac on 10 minutes later the signal was back (mobile and wifi) and is still on after 30 minutes where it has been left on my desk.
The battery seems fine. It charges normally and seems to hold charge ok. I have opened the phone after a 'power off' slide, removed the screen up and lifted the logic board to check the battery which is firmly stuck down and not moving. The logic board doesn't appear to have been removed before because the 'do not remove' sticker was still in place and undamaged. I replaced the logic board and tightened the screws properly making sure all the connectors were in place. No damage visible to the naked eye. Still exactly the same problem.
I am at a loss as to what to do next. Anyone have any ideas?