Default [Q] Bricked LG P970 or broken motherboard

Hello

E recently bought an LG Optimus Black (P970). It had stock rom 4.0.4 and the guy told me that it has CWM recovery on it, but i never tried.
It was working well, but one day it started to freeze quite often. One day i inserted another sim card than the one i was using before and it freezed. Restarted the phone but it froze again. The third time it didn't even boot, but instead the green android appeared with his belly open and the exclamation mark hanging out of it.
I tried a factory reset, but the process lasted more than 15 minutes so i turned off the phone.

Since then, when the phone starts the following happens: LG logo appears, Android guy appears with the loading sign in his belly and after 3 seconds the lg logo again.

I was able to access the SWUpdate menu and to flash a stock ROM with LG MobileSupportTool. When the update process ended with success, the phone should have restarted itself but it didn't. Instead the lg logo appeared again.

Trough the SWUpdate menu i managed to flash other stock ROMs on it with the SmartFlashTool but the same thing happened as before. (Successful flash, but the phone didn't restart. I even managed to flash a ROM on the device which had CWM recovery patched to it.

After this i created myself a 3in1 service cable with 56kohm,130kohm,910kohm resistors. I managed to access the SWUpdate menu with it, so i knew the cable was good. But i couldn't read from the device with CDMA tool. CDMA made the connection, but couldn't read.

I took the phone to the service, but they told me that the phone didn't respond to the JTAG either. They assumed that the motherboard is broken.

I didn't make peace with the idea that the motherboard is broken. I need help. Currently my only option is the take the phone to another service and hope that they are more capable than the previous ones.

Thank you in advance.