Lumia 800 (12070) won't power on - Infinite reboot...

Question

Hi everybody, 
I will explain my problem in 2 versions : 1 long, and 1 short. 
Short version :
Phone : 
Nokia Lumia 800 that I got sent by Nokia 2 weeks ago to replace my ex Lumia 800 which wasn't able to boot. (Different problem). Updated to the latest 1600.2487.8107.12070 version via Zune. 
Problem : 
Lumia 800 can't boot, it only vibrates and then turns off 0.5s after the operator screen pops. It does this in loop, random way, but sometimes it manages to boot. Whatsoever, it always turns off randomly, whenever, whatever I'm doing or not. Today it worked fine the half of the day, the other half was spent in rebooting.
At the beginning of the problem I was able to trying to boot it, holding the power button was making him reboot etc etc, but not anymore. Now, since today, holding the power button doesn't give any response. Only connecting it with a cable makes it reboot etc.
Symptoms : 
Totally random, I can't even know if the battery status is real, sometimes it says empty but seconds later it says full. Was working fine before yesterday. Nokia Diagnostic Tool doesn't show the Full Charge Capacity neither the information improvements that other are talking about, I have the 1.13 version I think. It is like it is still the old version. And it isn't always the same, sometimes it turns off, sometimes it reboots, but in all cases it is sudden, without "goodbye".
Trying to stay in offline mode doesn't worked, without microsim neither, neither soft reset, neither charging all night, neither fully discharging, neither none of any tricks worked.
It is my 2nd Lumia 800, and the 2nd that cannot power on. I'm affraid to send it back again to a Nokia Care, because of the 2 week I will have to wait.
Anyone, solution, help ?
Long version :
This is my 2nd Lumia 800, the last one was rebooting in loop, then it got stuck at the "Bouygues Telecom" (french operator) screen. No way to charge it or boot it. Got replaced by sending to the Nokia Care in 2 weeks. 
So they sent me this new Lumia 800, which I updated to the latest version : 1600.2487.8107.12070 with Zune of course.
Everything was working fine, until yesterday it suddently turned off while I was reading LeMonde app (french newspaper app). Note : No "goodbye", direct.
So I tried to turn it on, it vibrated one time, and once it got on the "Nokia" screen, it rebooted. Did this 3 times, then it booted properly. 
Later in the evening, I entered again in "LeMonde" app and it did the same, except it rebooted much more before booting succefully. 
I thought it was the app, so I uninstalled it, but it rebooted again while I was chatting, this time. 
Then I thought it had something to do with the network, as I had read things like this on the web.
So I tried to stay in offline mode (Plane mode) and it seemed to work a moment, except that in the end it did the same, loop reboot.
I charged it to my PC to boot it, no way, loop reboot. I disconnected the cable, tried to turn it on, it worked.
But it always turn off or reboot after a while, may be minutes, hours, etc.. 
Today it worked fine until 1h pm, it turned off, tried rebooting, no way, connected to the pc, no way too, was rebooting in loop, 10mins later it booted while still connected. Then, around 7h pm it began to loop reboot again. 
Now it is almost 11h pm and it still won't boot. It vibrates, the white "Bouygues Telecom" operator screen appears then it turns off (in 0.5s), and then it do the same, it never stops.
Note : Sometimes when I manage to boot it, it boot with the "in charge" indicator (the big one), but it isn't connected to any cable... Or sometimes it boot with the empty charge indicator (the big one) but then boot succefully and shows decent charge.
Note : Nokia Diagnostic Tool doens't show the full charge capacity, only voltage and current discharging, which is around 100mA, but can jump to + 200mA. Version of Diagnostic Tool is the 1.13 I think. Some on the web are talking about a new version with improved informations, but mine is still the old one, I can't understand why.
Anyone, solution, help ?
Thank you for reading.

Answer 1

well it turns out that I experienced the same problem here:
- have a Nokia Lumia 800, latest updates, French operator: Orange
2 similar UNWANTED REBOOT problems I have had for the past weeks, and mostly very recently:
- in France, it randomly reboots, apparently more especially when I travel (mostly in high speed train, but also underground or once in a big mall). I suspect it's caused by the device looking for a new BTS or losing signal, but it's not consistent. Sometimes, it just loses signal and my phone stays on.
for this problem, I could not find any solution, similar bug, something to help. I live with it, which is quite annoying but not killing me
- this week-end I was abroad (UK). In the Eurostar I mechanically set it to airplane mode, to prevent the battery consuming network scanning process. When arrived, I have tried to deactivate the airplane mode and, well, each time I turned it off, the phone asks me for PIN but quickly reboots before I can enter the PIN. Tried it with and without Wi-Fi, charged, plugged, entering the PIN code very quickly, without SIM card, underwater... no way.
I should have roaming activated but I don't think that is the issue. I have tried to deactivate 'time auto-update' but that did not change. I could not modify to Edge or 3G, neither any other operator parameter because... well because you can't while in airplane mode.
well clearly I was lucky I switched to airplane mode, otherwise it looks like I would have suffered from constant reboot... (i could use mine in Wi-Fi)
But NOKIA, please explain/fix this !!