Are you, by any chance playing in China?
Sometimes NPCs get stuck causing your game to stutter. This is very common in China, but can happen anywhere. Open the cheats panal - Ctrl-Shift-C and type: resetsim * then press the enter key. This will return your sims, and the entire town to home position.
If this works, then the problem is stuck sims. If it continues to happen, you will need to locate where they are getting stuck and remove the block. In China this is done by changing the problem lot to "no visitors allowed". Alternatively, it could possibly be a community lot which you have placed or a problem specific to the toen you are playing. It may just be a one off stuck sim too or a place with just occassional problems. Example, I have found horses stuck in the fence of the Science Facility in Sunset Valley, it is just an occassional problem, so, I don't do anything about it except resetsim * when it happens.