Problem I really need help with

[Radec]

I am using a Radeon hd 7870 GPU.
For a LG 1440x3440 21:9 display.
My GPU only supports HDMI or DVI so I can only get 50hertz for now.

My issue though is flickering, whenever windows are dragged, or any movement on screen will cause flashing and flickering.

However, when I plug in a second display (1080, 24"). It fixes everything, and my 1440 display runs flawlessly. Which is odd.
Now I no longer have a second display and cannot figure out how to fix this.

Does anyone know what might have changed when a second monitor is being used?
Any help at all would great.

 

[Radec]

Or even some help on how to search for help?
I'm trying to be specific, but searching leads to things that aren't helpful.

 

[Radec]

Just enabled a VGA out, screen went black, and popped back perfectly.
I guess I'll have to live a tiny gap on the screen where the mouse might get lost; But at least the screen is back to normal. So thank you!

And I will try and force the clocks up.
I tried moving speeds around with the AMD catalyst control centre. But that never seemed to work.