khalcyon

If you're using a save game to keep trying to go into the elevator, the save may be corrupted. You may have to start a new game. If you uninstall again, you will probably want to delete the save games too. Then reinstall and try it again. It's not the most favorable option to have to take, but at least you're only at Peragus and not further along in the game.

The ATI card isn't the only one that has this problem. My nVidia card does the same thing. I'm running on the lowest settings that I can right now. I also notice that it only lags like this when the PC is the lead character. When you use an NPC in the lead, the lag is not there. If you continue having problems try having one of the other characters in your party take the lead for a while and see if that clears it up.