getting vido acceleration error using external display, any ideas?

hi i have an imac intel 17" the lcd was lines gallore and unusable so i disconnected the lcd and inverter from the board. i now use an external monitor. when i try running a game i recive an error message saying

"video hardware acceleration is currently disabled due to safe mode operation or a driver error"

is there a way to fix this or do i have to leave lcd in and hooked up and just mirror displays? thanks - taylor