Like I said mate, I have exactly the same problem with SoA... it just consumes so many resources and the game starts to crawl.
At the very worst times, the computer actually crashes. Sometimes I have 20 second hard drive accesses to save the game. Not good eh
The strange thing is, the real slowdowns can be somewhat sporadic... some days there isnt a problem, other days... it really crawls.
It could *possibly* be due to a slightly dodgy fan on the graphics card.
New suggestion: Pop open your case, and keep an eye on the graphics card fan for a day or 2.