S3 won't boot. Or enter recovery mode

[AC Question]

I have searched for hours for an answer but with no luck.

My Samsung Galaxy S3 GT-I9300 rebooted by itself yesterday. Now it is stuck on the Samsung Galaxy S3 screen.
I can boot into Download mode, but it will not enter recovery mode. So I cannot perform a hard reset or clear caches etc.

I have tried to connect it to Kies with no luck.

Any ideas what I can try next?

 

[B. Diddy]

Maybe try a new battery? Also see what happens if you remove the SD card, in case it's corrupt.