There is a BIG difference between a little lag now and then, and unplayable frame rates, please be clear on what your issue is.
I have a 3 month old Slim.
Up to 2 hours playtime: Good framerate sometimes, bad framerate sometimes, terrible framerate in forests and some dungeons with heavy mist / dust effects. Stuttering is often but mainly due to texture loading etc, still often enough to be annoying though. Maybe once every minute if running around in the wild or a big town.
2 hours plus: Game stutters a lot, on average once every ten seconds, same as loading stutters, but all the time. In dungeons or towns in big fights game is basically a slide show. 5 frames per second perhaps.
I have however, had no game ending bugs, only 2 crashes and the only bugs I've had are very minor. BUT after 2 hours the game is borderline unplayable. Especially in hectic fighting.