its prolly the psu guy, 1200w would barely be enough, I have an amd rig that does the same kinda with 6 gpus, windows ends up totally freezing up, considering the rig is really dependent on one thing! power

first off msi afterburner isn't accurate on what it tells you as far as the power the gpu is consuming, its only accounting for part of the gpu
secondly seems you have forgot to account for the power the motherboard needs along with an additional 20 percent to account for power spikes from initial startups of the gpus and stuff

here is a little basic math 1000w/6 gpus is around 166.6 w per gpu
which might I add you didn't actually say if that 1000w was of the whole rig or just the gpus

now if that's whole rig id say bump that down to around 142.8w to account for the power the motherboard takes which could be an upwards of up to 150watts

risers also need power too

with just the info you've already said I would have to say its the psu being not sufficient, get a 1500w psu