I've encountered the same problem this morning, my iphone 3GS restarting itself 2 to 3 minutes and it freaks me out.I tried restore it, during my first restore I've received an error message saying back up failed but I've go for the second restore immediately without backing up and it works perfectly again.Don't restore from backup, try restore from new.Hope it helps.
Try DFU Restore,
1. Connect your iphone to your computer.
2. Shut down the your iphone.
3. Press the Power and the Home buttons at the same time.
4. After 10 seconds release the Power button but continue holding the Home button
until you see a popup from iTunes saying "iTunes has detected an iPhone in recovery mode"
5. If you have been successful your device screen will be completely black/blank (this is 'DFU Mode')
6. You can now proceed and restore to the latest firmware using iTunes
(P.S:Before trying this please google DFU Restore and understand exactly what it is for I have never tried this before.Goodluck!)
Guys.. the problem happend to me yesterday.. it happened after upgarding the iphone to 5.0.1.
The Solution is:
1- Remove the SIM card from your mobile
2- Enable the (3G & cellular Data)
3- Add the SIM card again.
4- Now .. it will working fine.
Also you can Disable the (3G & cellular Data)
I hope this fix your problem..
EDIT: see final comment...
Unfortunately the SIM removal didn't work for me. It still is rebooting every few minutes.
I have 3GS, 5.0.1, and had data turned off, but 3G turned on.
It was all fine until this morning when my battery went flat, I put it on charge and after a few minutes turned it back on (charge was at 8%). It didn't turn on by itself which I thought was odd.
And since then, it's been restarting.
I turned off 3G, still reboots,
Removed the SIM, enabled 3G, replaced SIM, still reboots.
Any other ideas?
EDIT: Ok, after the final reboot above, I got a flood of texts when it restarted, and since then it appears to be ok.
So, EPIC KUDOS to saadalsakran, this fix does work, but you may need to try a couple of times.
Message was edited by: jjkboswell