Samsung Galaxy S3 not booting up, keeps restarting showing ”Samsung Galaxy S3 GT-i9300” black screen
Enter recovery and delete dalvik/cache. To enter recovery you have to hold Volume up + Home + Power (The same with Volume down is download mode). Once in recovery, select the option to wipe cache/dalvik, then restart the phone.

If the phone keeps rebooting as it was doing before, try searching the log of the boot sequence to see if you can find where it fails. You will need a PC with Android SDK installed and the phone's USB cable.


With the phone off, open a CMD window in your PC, go to Android SDK directory and run adb logcat
Turn on your phone and connect the USB cable.
You should en seeing in your cmd window the log of the phone as it boots.
Once it enters a boot loop, you should be able to check the log and see what it tries to do and can't do (that triggers the boot loop).