Thank's to @Gogito telling in the Q&A thread that increasing voltage fixed the reboots, it made me look again at this dear frequency table. And there I thought : "What if the problem is not the voltage of the 1.8GHz frequency (which I already put really high), but the voltage of the 281 MHz frequency ?"
So I tried to increase it a little bit aaaand there it is, I don't have reboots in Antutu anymore !
Since my phone seems to be a little stronger than yours (it didn't suffer from random reboots on normal use), I'll send both of you a test version that you can try. Please tell me if the reboots are fixed for you too !
Btw, I get over 30000 in Antutu with stock rom !