PC: Bad Company 2 lags/judders, Vietnam Expansion Pack NOT?

Have been playing BFBC2 for over a year, no problems (everything on low, though). A couple of months ago, it suddenly had a noticable decrease in performance: slight lags, juddering, reduced FPS - just as if my hardware wasn't good enough. The thing is: the expansion pack "Vietnam" runs just fine! How can this be? It's the same game, same engine, etc. Any ideas?