[aculaj]

If your N8 is restarting, try to connect via USB with PC and see if it still restarts when it's connected with PC.

My N8 restarts very often but when it is connected with pc it doesn't restart anymore.

Try this and let's see if we are getting close to find the problem.

[carman58]

If your phone is restarting and freezing it needs returning to Nokia for repair or replacement, not connecting to a computer via usb cable !

[aculaj]

In my country there isn't any nokia center around.

[carman58]

Then contact whoever supplied it, the phone has a minimum 12 month warranty and hasn't been released a year yet, so return it for repair or replacement !

[aculaj]

The phone was bought in USA, NY but it has no warranty because it was bought from mobile shop. I've checked for warranty online on nokias page, but it told me:
We've now checked the warranty status of your Nokia device and found

Sorry - according to our system, the serial number 356************** you have entered is no longer within the warranty period.

[carman58 · Accepted solution]

Try this, the phone can't be a year old so you WILL have a Nokia warranty 

http://www.nokia.co.uk/support/repair/repair/warranty

[aculaj]

I've already tried this and it says:

Sorry - according to our system, the serial number 356************** you have entered is no longer within the warranty period.

[synth_fg]

How long is a standard US warenty ?

Anyway could be an issue with the memory card, try removing it and seeing if the phone becomes more stable

If not try a factory reset,

failing this take it back to the store you bought it from / a nokia care point and see what they advise

[aculaj]

I have tried everything, Factory Reset, Format, Firmware Update (now is PR1.2 014.002). Tried with and without memory card, with and without sim.
Only when it is connected via USB with PC it doesn't restart.

[carman58]

http://nds1.nokia.com/files/support/nam/phones/guides/US_standard_phone_warranty_en.pdf