Phone boots very slow and restarts by itself My problem is when I turn on the phone, the Google logo stays stuck and after unknown minutes boots in with the software animation, but very slow and then starts over again! First a little history of my upgrades with this phone.

My friend bought me Nexus S T-mobile version for US 6 mounts ago, and the phone was with Gingerbread 2.3.4. After reading many post from xda-developers , I decided to upgrade because I can't upgraded 2.3.6 OTA. After many hours I finally upgraded it to 2.3.6. 
Then ICS came out, and again, I upgraded to 4.0.3. Then, a few weeks ago, I got the 4.0.4 OTA notification. Happily I taped yes. And then before two days ago, my phone just died. I don't know how and why.

I have 115 installed application, no music inside, no ROOT - just pure stock. I was in my college room and I left my phone on the desk right in front of me and after few minutes when I looked at the phone, the Google logo was there. I didn't give the phone to anyone to picks or anything. When I get home, I turn the phone on and still the Google logo just was there doing nothing and like I said before, after unknown minutes the phone boots in with very slow animation of the ICS software. I decided to install Clockwork Mod recovery 5.0.2.0 and root the phone. After reading many tutorials I installed CWM recovery successfully. Then I tried to downgrade to 2.3.4 and successfully I did that to. 

Now after doing all this things to this phone, after so many hours in pain, the phone is nothing like one week ago. The booting is very slow, the animation is veryyy slow, and I don't know what to do anymore... I think this is hardware problem. 

[edit] I send it to Samsung and they send me a new one, so definitely is a hardware problem.
After flashing a new firmware it is advised to perform a factory reset (all your data will be lost, so take a backup.) While the phone is off:


Press the VOLUME UP, HOME and POWER buttons together till it vibrates. Now it has entered RECOVERY MODE.
Navigating using the volume buttons highlight the option wipe data/factory reset and press the HOME button to select and confirm it on the next screen.
Finally select reboot system now to restart the phone