Constant Window Lag on Windows 7 on High Spec PC

I have some displays issues with my pc. Whenever I open a window or move it around, the screen lags and leaves a trail of the window in its path. Its hard to describe, so I created a gif which shows the problem (sorry for the rubbish quality):

http://imgur.com/yvJeBNL

I've had this problem now for a very long time (probably a year), but just ignored it because I could figure out the cause. I thought it was a graphics card issue. However I've downloaded, uninstalled and installed all the latest drivers many times and it makes no difference. To add to this I can play most games on full setting with no problems or lag. I'm not sure why this is happening. Any help would be appreciated, its getting very annoying now.

System Specs

OS: Windows 7 Professional N
CPU: Intel i7-4790k 4.00 GHz
Memory: 16gb RAM
Disk: 3x Samsung SSD 840 EVO
Graphics: Nvidia GeForce GTX 980
Display: Asus 27" PG278Q G-Sync monitor