In the drawGame function, delete the pygame.display.update()
That will fix it for ya.
It was like that because you had 2 pygame.display.update() functions in your "while not gameIsPlaying:" loop. So what happened is you drawn the game, flipped the buffer, then drawn the text and flipped the buffer again, so text and game were drawn at different buffers. So when it updated, it did it really fast, and you saw different buffer every frame, and that caused the flickering.