LOW FPS ON FULLSCREEN?

Please, someone help me, i cannot play correctly since this issue happened.

Well, at least the problem started with this new deskop PC, wich im improving it for games, before i played in a laptop wich is a notebook and NEVER got this issue before.

simple, when i put the game on fullscreen the fps drop to 30-25, no matter if I put the most low resoultion - to the highest, the FPS didnt got fixed, touching lag compensation in config, neither, i tried to change the Hz of my monitor (from 75Hz to 60Hz), also nothing, i did a verify on the cache of the game, did nothing, sometimes it got  60 fps at the start, but i cannot even start touching an option of the menu, because they drop to 30 again,

my specs:

Intel core i7

8 GB ram DDR3

AMD Radeon HD 6450

Monitor of 75Hz

Windows 7 (64 bit)

please help,

i already ask to the support, thank you for reading.