Question regarding Crysis 1 Performance

So, this isn't urgent question as i already finished the game a while ago, but i was curious. Back when i played it, i was able to max the game out on an HD 6870. Now halfway through the game i started experiencing crippling lag, at first it seemed random, then it became more constant. Near the end of the middle what would happen was that after booting up the game i would be able to play with no lag for up to 20 minutes before shit started hitting the fan again. The only way to fix it was to reboot the entire computer, which usually gave me another 20 minutes. Near the ending that 20 minutes became dependent on how much action was going on the screen. The more action, the faster it would move from performing well on max to being unplayable. This is all in single player of course. I never tried the multiplayer. So when i say lag i mean major drop in FPS. To the point of almost being unplayable.

So my question is, why did this happen? Did it have something to do with the RAM? To clear things up, this only happened in Crysis. No other game became laggy 20 minutes in or halfway through the game. It was only Crysis. I don't really understand what happened. And i tried to explain it as best as i could. If anyone has any idea of what happened i'm curious to know.