It's possible that the game has outpaced the graphics card in terms of specification, Jonah. These days, a lot of games require a 128Mb card, which I found out when playing the Harry Potter games, even though they don't say so on the box. Also, a friend had problems with Colin McRae Rally 3, before upgrading to a 128Mb card. Perhaps the game manufacturers don't want to put people off buying them in the first place, in the hope that they won't notice graphic intensive parts of the game suddenly running in slow motion? What does it say are the minimum requirements for the game?
Obviously I'm only guessing in your case, but do, however, speak from having similar issues, which were cured by a graphics card upgrade.