help with playback!

Hi!

this is the first time i post here..

i have a pc that i have built myself


ASUS P9X79 PRO - ATX / Intel X79
Intel Core i7-3820 - 8 trådar / 3,6GHz (3,8GHz Turbo) / 10MB /
Corsair Vengeance 16GB DDR3 PC3-12800 1600MHz
Samsung 830 Series MZ-7PC128 128GB (SSD)
XFX Radeon HD7770 1024MB


I have downloaded XBMC and use aeon mq4

But when i play .mkv and full blurays it lags a little bit...
not much, but enough to become irritating!
what can i do to make it playback smoother?
is my graphic card to slow?...but it is gddr5 and 1 gb?

i think it is strange that i can play flawlessly in netgear neo 550 (mediaplayer) but not in my pc...

I hope that someone can help me...if you have questions i will be happy to try to answer them....and sorry for my band english..