Quad Crossfire and 120hz

[GotNoRice]

When using my 2x 4870x2 in Quad CF, I get display corruption at 120hz. Everything is okay at 60hz, and even at 100hz, just not 120hz.

Single-GPU operation is okay even at 120hz, just not with 2, 3, or 4 GPUs.

I'm using the 12.6 Legacy drivers with 12.8 CAP3 installed on top. My 120hz monitor is a Samsung S27A950D.

Any ideas why this would happen?