The thing is, even though it worked fine for the base game, each ep adds animations which make more demands upon your computer, particularly your video card, especially from Nightlife onward. If you go to the sims2 site and look at the requirements for the base game, and compare it to all the eps from university onward, you'll see the basic minimum requirements for just about everything change for just about each ep.
This increased demand on computer resources is why so many people find everything ran smoothly with the base game, but not with the eps.
Information on all the eps and stuff packs Click on "learn more" then "System requirements" to find the specs.