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Hi @Kredenn,
Sorry to hear about your phone.
I can understand that this can be quite an annoyance. There are a few things we can try to sort this out. As @Samooorje, a software repair using Xperia Companion is one option but considering that will reset your phone I'd recommend it as a last option.
If the phone keeps rebooting that often, can you try (temporarily) removing your SIM- and/or SD-card followed by booting your phone. Are the restarts still occurring?