Very Low Frame rate on high resolutions

[berkinster]

Hi there;

I got AMD 7870 gpu and Intel 3570k cpu with 8 gb-ram.

When i change the resolution to 1920x1080 geting 10-15 fps even lowest grafic settings.

but in low resolution like 1080x720 or 1680x1050 getting 45-50 fps in very high/ultra settings.

I have the latest gpu driver. any one know , how can i fix this problem?