Experience with M55S-S3

[Systemcheck]

Hello all,

I cant Setup more then 1 GPU to my M55s-S3.
My OS is Win7 64Bit, cpu amd Dual Core (2x2,8 ghz) and 6 GB RAM.
I have 4 GTX 1080 connected with pcie 1x risers (powered). But only 1 gpu run in this Setup. When i connect 2 gpu After a des minutes mining i got a Display reset error and only 1 gpu is working again.
Whats Wrong there?

If Anyone using a setup like this let me know your experience.
Cards working on another mobo Fine.