I don't think he could have tried harder in those instances. I think he effort in those games was as high as it possibly could be. I just think that, in the end, talent and decision making are going to win out over all the other things. No matter how hard he tries, from time to time, and much more frequently at this stage of his career, Brett Favre is going to make an awful decision.
Also, when did I say he doesn't show up in big games? However, for what it's worth, I can show you a fair amount of playoff games where Brett played poorly.