Extended cut freezes and choppy framerate

Hi,

I've been an avid fan of Mass Effect trilogy for 5 years without any issues. Mass Effect 3 has been running smoothly until I installed the extended cut(DLC). When I get to the citadel and start talking to the Godchild, the game starts to slow down. On the cut scenes,  the audio portion is working fine, but the framerate starts to get choppy and then game freezes.

I have: re-installed the game; updated the drivers; changed settings in the config utilities to low; turned off vsync;  I unchecked enable origin in-game  but the problem still exists. I have spent hours and hours reading the forums, checking online with no solution. Please help me with this because I really want to play this fantastic game to the end.

System Information
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Operating System: Windows XP Professional (5.1, Build 2600) Service Pack 3 (2600.xpsp_sp3_gdr.120821-1629)
Processor: Intel(R) Core(TM)2 Quad CPU    Q6600  @ 2.40GHz (4 CPUs)
Memory: 2048MB RAM
DirectX Version: DirectX 9.0c (4.09.0000.0904)
Card name: NVIDIA GeForce GTX 460