6680 turns off automatically

[threat]

a brief overview of my problem:
i have this 6680, which i have kept in very good condition.
last year i tried an experiment of flashing it with n70 firmware with usb cable, but of course it didn't pay off
i was on vacation and didn't had any equipment with me, and also didn't make a backup of pm
so i gave to a repair shop, he wrote new rpl and then firmware.
it seemed to be good again.
but then this strange thing started to happen: it turns off*** automatically at irregular intervals, whether i am using it or it's in idle mode.
*** it doesn't turn off in normal way, the screen just straightaway blacks out.
another inetresting thing is that if the keypad was locked before this abnormal turn off, than it is still locked after i turn it on
also, when i put it to charging, after some time the phone freezes, i.e. the screen turns to low backlight mode and no buttons work, the battery signal still keeps running, and then few minutes later it turns off (but this time in a normal way), and it keeps charging after that in off state
i have flashed it with jaf, phoenix and nsu, with different firmwares but to no avail
so, it seems that it's not a software issue or atleast not a firmware issue.
also, without any reason i scanned it with BitDefender Mobile Security v2 and it reported that there is a virus in my ROM Z:/.../startup.ro2, it says it's infected with Symbos.Warrior.A
so, the situation is still the same and local repair shops can't repair it
HELP!!!