Dell 3415W - DP / mDP don't work

Hello,

I' ve just bought a monitor Dell U3415W.

I have a PC with an AMD Radeon 290.

If i plug an HDMI cable to  the monitor.. no problem (but games run very sluggish, maybe because of the 50Hz refresh rate, or maybe lower)

If I plug an mDP cable to the monitor (the original mDP cable found in the box), the display flashes (screen disappears then appears randomly and very often, but games run smoothly)

If I plug a DP cable to the monitor, it doesn't show anything and goes in "energy saving mode". I have to reboot my pc in "safe mode" to see the screen again.

I've tried all AMD Drivers available.

If i install the "Microsoft Basic Display Adapter" drivers, all the cables (HDMI, mDP, DP) seem to work fine (but obviously, no games will run with those kind of drivers...)-

What can I do ? :)

Thanks