6x Powercolor Red Devil RX480 8GB crashing

Hello, i am having issues with one of my rigs.

Hardware setup:

Motherboard: Z97X-GAMING 3
CPU: Intel core i3
Ram: 2x 8GB Kingston
Hdd: 1x 128GB Kingston
Psu: Leadex Titanium 1600W
Gpu: 6x Powercolor Red Devil RX480 8GB
Risers: PCI-E 16X to 1X Adapter USB3.0 USB 3.0 Riser Cable Flex Flexible Extension Cable w/ Molex 4 Pin Power Connector

The problem is that only 4 of them work, always 2 of GPU's after working for a few seconds they drop and they don't mine anymore.

I have tried both Windows and Linux with Claymore, same issue with 2 gpu's dropping after a few seconds.

Any advice would be helpful.

Thanks.