Help Sound/Reboot problem, need opinions

[Daniel Tom]

Hi, i have some problems with my galaxy s3 (i9300) international version. I don't have a warranty.
Android version: 4.3, stock
Problems:
-keeps restarting (very often)
-no sound at all, speaker or earphone
-microphone not working (not working in call, google now said that cannot connect to microphone)
I tried reflashing stock, same problems, i tried 3 custom roms including cm11, same problems. I cleared cache, dalvik, wipe data, nothing changed.
In my noob opinion, i think the motherboard is the problem, i don't think it's a software issue after all i've tried, but before i order a new motherboard for replacing, i would like to hear other opinions.

Thx in advance.

 

[BynoUK]

All signs point towards a hardware failure, you've already carried out the steps I'd have suggested too. Hopefully someone has a solution but IMO it doesn't sound good

 

[Daniel Tom]

Thx for the reply, i will replace the board if i will find a cheaper one. Sadly, at the price of a board i can buy a new phone.