I found the cure for OVERCLOCKING 1055T on 880G@890GX. There is no NB voltage! OC Tuner is a LIE!

In bios there is something called mGPU voltage. Since GPU is the same chip as NB and it's stock voltage is 1.4V ... I've bumped this voltage to 1.5V and overclock improved a lot!
Still testing but this may be a clue for all bad 1055T ocing on this mobo.