Crossfire problems with HD5770

Hi,

Im trying to setup 2x 5770 on my system (im adding a 5770, already had one)

I installed both cards using 2 PSU connectors that were avalaible (i could also use PCI E modulable connectors on my PSU but it seems i already have those connectors so i dont really need to add them)

When i started my computer, the card was detected and installed succesfully.
I tried playing CS GO and i noticed it was laggy...graphics were at high wich is not surprising but it was unplayable...i put them to low (as it was before with just one card) and still i was laggy (ex: when in front of smokes mouse is shaking because of lag and i cant even aim) i never had this problem before.

I decided to flash bios and also upgrade cataclyst to version 14.501.
Problem is still there, it changed nothing.

I tried Black ops 2 and i have the same issue, it worked better with just one card!

Here are my computer specs:

i7 860
Intel board DP55WG
2x sapphire radeon 5770 1GB (no OC)
4GB Kingston hyper X 1600 ram
Thermaltake purepower 750 watts PSU
1tb Sata WD

What's wrong there? CPU too weak?...memory not enough ?...

Please help me ! :P

Thanks

Crossfire problems with HD5770