I've played all the Tiers and all the boss except Firestorm Citadel and Vanaduke.
Tonight I decided to go into FSC. First part of FSC where you have to dodge the rolling things and go through a fire maze, finished without issue and continued on.
Second stage where you have to rescue six of those thingys for water balls... I started twitching like crazy. I'd freeze up for 1-3 seconds, but my character continues walking. This wouldn't be such a problem if there weren't fire pits and skeletons everywhere.
I'm running on Windows 7 Home Premium 64 bit
Nvidia Geforce GTS 450
6GB ram