I have a 3GS phone that was on 4.1 and not jailbroken. The phone would boot up fine and everything works but after 5 min it will reboot itself. I have the shsh blobs saved so I tried restoring it to 4.1...same issue. I then tried restoring it to 4.2.1...same issue. Now it is on 4.2.1 and jailbroken with greenpois0n...same issue. Everything I read when people have this problem says do a fresh restore to fix; I've tried that multiple times and it still isn't working...???