Can not get 144hz with HD6950 on AG271QX monitor

[Allsider]

When I connect it with DisplayPort, the max refresh rate I can see and "set" is 120hz. But when I set it, the monitor says "no signal". The refresh rate in monitor menu says H.Frequency: 88hz V.Frequency: 60hz, with 2560x1440 resolution and H.Freq: 67hz V.Freq: 60hz with 1920x1080. I can't try with DVI-I cable since the monitor only has DVI-D connection (I did try with DVI-D, no luck).

Motherboard: ASUS P5E, PCI-E x16.
GPU: XFX Radeon HD 6950 2GB http://www.amd.com/en-us/products/graphics/desktop/6000/6950
OS: WIN 10 Pro
Monitor: AOC AGON AG271QX http://aoc-europe.com/en/products/ag271qx

Please help me figure out what I can do to solve it. I have checked all the monitor settings, and they seem correct. Re-installed the driver as well.

EDIT: When I change the monitor setting to DisplayPort version 1.1 instead of 1.2 (or change from 8 bpc color depth to 6 bpc), I see the 100hz option and it seems like it works (monitor menu displays H.Freq: 151KHz, V.Freq: 100Hz). Still need 144hz though. On the HD6950 product page it says that 1.2 DisplayPort is supported. I wouldn't mind using HDMI, and I have tried, but the 120hz option only works on a lower, unusable resolution.
In the User Manual for the monitor only AMD Radeon R7 and R9 series are listed as supported monitors... So what does it mean for me?
Can it simply be that the monitor driver doesn't support my GPU?

@spat55, Yeah, I have now ordered a new cable just to be sure. The length is the same as the one I have now, 200 cm or about 6'7".

 

[spat55]

I've had an issue before with Displayport not showing 1440p/144hz and the reason why was I had a bad quality DP cable, I switched it out for another and I had no issues, length seems to play a part in losing some bandwidth which is needed for higher refresh rates and resolution.

 

[Allsider]

I noticed now that connecting through DisplayPort does not work without connecting my second monitor (DVI to VGA connection), or having the second display enabled in the display settings (after disconnect). I'm not able to use one monitor with DisplayPort (No Signal message). I will update this further when I get the new DP cable.

edit: I upgraded my ssytem with a RX 480 card, monitor seems to work at 144hz now (DP to DP, not miniDP to DP like before). (cancelled my DP cable order)