It's possible you have a faulty battery.
I was convinced my GN2 was a write off. It just showed the gn2 splash screen for a few seconds, before restarting and doing the same thing every 10 seconds. This was without Mains connected. Battery didn't enter my head because even though the phone is over 18 months old, battery life has been good - 6-8 hours.
Plugged into Mains, I got further, but not much. The phone would start to load, but would often restart during this, and would also often just turn itself completely off and show the green battery screen. Occasionally I'd get right through to the home screen and could use it, but every few seconds / minutes it would suddenly drop back to the sim pin screen, and would still occasionally reboot / turn off. It was also doing strange stuff like forgetting my wifi key or reporting corrupted files on the SDCard.
I suspected the battery at this point, but was unable to prove anything without a different one to try. All I knew was that it wouldn't start without Mains, was still flaky with Mains, and would turn off as soon as I pulled the mains out. The phone won't turn on at all if the battery isn't present, ie won't run on Mains alone.
Turns out not all chargers are created equal, and I'm not just talking about amps. The charger that shipped with the phone, while powerful, appears to charge with a variable pattern, ie high power for a few seconds then low power. If it's quiet, I can hear the circuit buzzing, and changing in intensity. I've found that with a dumb 0.7A Nokia charger that the phone is completely OK, as long as it's connected. Now that the phone is stable, I've been able to monitor the battery more scientifically, and I've found that while it reports 100% charged, it still turns off straight away when I pull the plug, and if I plug in and boot up right away, it shows 10% charged, and charges very quickly (from 10% to 70% in 25min).
So, in summary, it looks like a suddenly dead battery, and a charger with an unhelpful charging method, are responsible for a lot of strange behaviour. I've got a battery on the way, and I fully expect it to restore my phone to full performance.
Just don't discount battery as a possible fault, and DO try a variety of compatible chargers, is all I'm saying. Hope this helps someone, if not the OP.