There are several possibilities.
Sometimes, especially if you are playing in China, NPC's get stuck. This causes the game to stutter as you describe. Try typing bringing up the cheats bar and typing resetsim". This will reset all sims, including your own to their home positions and you should be able to play normally, at least for a while. If it continues to happen, you will need to find the place where the sims are sticking. There is some bad pathing in China which can easily be avoided but it requires a one time only cheat.. I don't know if there are major issues like this in any other Worlds as, apart from the WA destinations I generally only use Sunset Valley. This happens occassionally in SV when horses get stuck in the science facility.
If that is not the cause, try reducing all your player options to the lowest level. (Draw distance etc.)
Are your computer specifications adequate? Particularly check your graphics card, you may need to buy a better one.
