iPhone suddenly - right after a perfectly fine phone call - rebooted and went into a cycle that after about 10 seconds, would shut down and start again. Tried charging the battery. Tried re-installing the software. Everything failed to get the phone out of the cycle. Never once progressed to the start-up screen. Ordered new battery.
Everything went perfectly well. iFix it doesn't address the antenna which is screwed into the same place as the battery screw, but it's pretty obvious to take out and put back in. Just don't be surprised when it suddenly appears to come loose. After the repair, had to put the phone through a full re-load of software and back-up data replaced. It took twice to get it working smooth, but since then absolutely no issues and it's like the phone never had a problem.
If your phone is turning on, booting half way, and re-cycling - do try to replace the battery. It was a $50 investment with battery, tools, and shipping, but worked like a champ. Problem solved and life goes on. Replacing the phone would have been much more expensive and complicated.