Problem found : i'm an idiot! 
When i assembled my pc, i inserted my videocard into the lower pci-e slot of my motherboard. It seemed like a good idea to prevent heat buildup between videocard and cpu.
So i assumed those 2 ports are the same, but apparently the upper on is a PCI-E-X16 slot, while the lower one is PCI-E-X4
I moved the card, and i now play at 1024*medium in DX11 mode with all fancy features enabled, and the frames bounce between 50 and 100!
When i assembled my pc, i inserted my videocard into the lower pci-e slot of my motherboard. It seemed like a good idea to prevent heat buildup between videocard and cpu.
So i assumed those 2 ports are the same, but apparently the upper on is a PCI-E-X16 slot, while the lower one is PCI-E-X4
I moved the card, and i now play at 1024*medium in DX11 mode with all fancy features enabled, and the frames bounce between 50 and 100!
