Random iPhone crashes and reboot loop after iOS 7 upgrade

Question

Hi all
Since I upgraded my iPhone to iOS 7, I suffer from random crashes/resets, where my iPhone reboots and enters a reboot loop (Apple logo, black screen, reboot, Apple logo, black screen, reboot, etc) that I can only exit by restoring the device (resetting doesn't help).
Some more information
a) The device: iPhone 4, 32 GB, has never been jailbroken, and has never shown any similar issues before.
b) The crashes/reboots: I haven't been able yet to find a pattern when and why they happen. I had reboots when searching for an app, when pluging in the wall charger, when entering the access code, and even when doing nothing at all (iPhone in my pocket). The only constant is the reboot loop after the crash.
c) What I tried so far: I restored the device several times, using normal restore (with the iTunes logo) and DFU mode (black screen). After that, I recovered from a recent backup (takes some hours), and everything is ok again - for some hours, until the next crash and reboot loop. I also set up the device as a new phone, and there it seems stable, so I exclude a hardware issue. I tried quite a few of the advices that I found when googling for reboot loops: reset all settings (this led to an another immediate crash and reboot loop), disable app background refresh, etc.
I've been using this device for my daily business. With the current instability (and the couple of hours it takes to bring it back to life again), it became more or less useless. I wish I could downgrade again (I know that I can't). My assumption is that either iOS 7, or some incompatible app or some corrupt setting is causing this situation. But I don't know how to proceed to isolate and eliminate the issue. I really would like to avoid setting up everything from scratch again.
Any advice is greatly appreciated!

Answer 1

All,
First of all, sorry for my half-finished post above. While typing, I was interrupted, and by reflex I hit CMD-S. Unfortunately, the stupid interface of Apple's support website interpreted that as a submit command. I tried to complete my text with the update function, but when finally hitting the submit button, I got an error message, and all my updated text was gone... No way to bring it back, and also no way to delete the half-baked posting. Quite frustrating.
Now, to complete what I started above:
In my case, the problem definitely had something to do with the springboard process (an iOS system 'app' that manages the iOS homescreens) and the watchdog process (another iOS system 'app' that monitors certain things on your phone and that can initiate a restart when it thinks the phone is frozen). In the folder '/Users/<your account>/Library/Logs/CrashReporter/MobileDevice/<your phone>' I could see what caused my crashes; they were all 'stack+SpringBoard' and the reason in these crash log files was 'Watchdog timeout'.
The following two forum posts helped me:
https://discussions.apple.com/thread/1676330?start=30&tstart=0
https://discussions.apple.com/message/20794982#20794982
I'm quite sure that my issues had to do with the 'application map' that is mentioned in the above posts. So far, Ive been thinking that it was just too large (as in the case of Andi1996 [second link above]), but jmbladen's observation seems not to support this. So, the absolute number might not the source of the problem, but I still assume that something went wrong with my application map during the upgrade from iOS6 to iOS7. I remember having stability problems before, as soon as all my 10 home screens were full of apps and newly downloaded apps would generate additional pages (only visible via iTunes). Considering that there are some fundamental changes to how we can organize apps within folders in iOS 7, it can well be that the springboard process just wasn't able to cope with my application map correctly within the 120 seconds before the watchdog process restarts the phone. My massive cleanup action must have fixed this somehow.
@jmbladen: I'm only using iCloud for the 'find my phone' functionality, so all the backup & recovery was done via iTunes from my local harddisk. And there was no temporal pattern for my crashes before. You might suffer from the same symptoms, but probably not because of the same cause.
My advice for anyone affected by random crashes and a reboot loop is to check out the directory with the CrashReporter logs mentioned above to find out what app or process causes the crash. If it's the springboard and watchdog problem, then you should try to clean up your home screens (from iTunes). Maybe the absolute number of apps is not that important, but make sure that you reduce the number of screens. And then sync and backup before you start using the phone again. In my case it helped.
Hope this helps someone else too.
P.S. Once you know the right keywords to search for, you find forum postings back to 2008 / iOS 2 describing exactly the same problem. This reboot loop is a very nasty issue, and it is hard to understand that Apple still hasn't been able to address this, or at least provide some better help on how to get out of it again.