Update you graphics card drivers to the newest one avaible for you card.
also make sure you dont have someting like cool en quiet on in the BIOS it might downclock your card because of its temperature and kill your fps by doing so.

Hello,

i bought chivalry on the steam sale played for 3 days but now every time i join a server it says Authentication failed

restarted game
restarted steam
restarted pc
restarted router
and i veryfied integrity of game cache

but i still get the error every time i try to join a server really irritating.