You dont need a bigger PSU...700-800w is more than fine.
Maybe that one you have now is broken, test your system with OCCT.
The power supply is less than a year old and doesn't have any of these issues when using an R9 Fury Crossfire setup, or a GTX 1080. Also, games have load fluctuations (spikes in voltage) due to a lot of changes in GPU utilization compared to a constant load test like OCCT. Running Unigine or OCCT doesn't cause this issue. It happens with games like FFXV and PUBG.
Here are the total system power consumption for the PC for the various tests I've done. All games tested are at 1440p with freesync enabled.
FFXV 60fps cap -> 430W
FFXV 120fps -> 610W
FFXV 30fps cap -> 270W
PUBG Ultra 120Hz refresh -> 600W 80fps average
FFXIV 120fps cap -> 600W
FFXIV 60fps cap -> 550W
BF1 60fps cap -> 500W
BF1 120fps cap -> 580W
Idle desktop 60Hz -> 113W
Idle desktop 120Hz -> 115W
Idle desktop 144Hz -> 140W
Unigine Valley -> 515W 1440p Ultra 48fps average
Unigine Heaven -> 510W 1440p Ultra 61fps average
Found this forum post http://www.jonnyguru.com/forums/showthread.php?t=15653 over at jonnyguru talking about RX Vega 64 and my Seasonic PSU going into OCP/OPP because of Vega 64's transient spikes. Apparently total current spikes can exceed 85A which would cause the 1000w seasonic to trip OPP. If so, that's insane and it means that custom cooled Vega 64s and the Liquid cooled version should recommend 1200w units instead of AMD's current recommendation of 1000w.
Changed to a Corsair HX1200 psu and that seems to have resolved the issue.