I experienced this one for almost a week. Irritated and frustrated. I can't play anything for a single minute. Asked LG how much is it for the mainboard and they cost me Php13,000 (more or less $200). But right now, I just made my own fix for this.
Downgraded from MM to LL (stock, rooted), installed CTT mod, installed Xceed kernel, underclock CPU to 1497.6MHz, undervolted to -100mHz, undercurrent to -8mA, underclock GPU to 389MHz, turned off Knock On/Off Setting.
For more than a week, I can play AAA+ games like Implosion even while charging without any problems. I just noticed for G3 phones that it easily throttles its CPU to maximum clockspeed (2.6GHz) when it reaches 60degrees of temperature and causes our phones to overheat together with the LCD temperature and makes our screen to flicker.
CTT mod does everything for me. It lowers the temperature that my G3 has.
Hope this helps.