I'm having a very similar problem to what was mentioned in this thread, only since I installed the latest game pack (Parenthood). My sims will freeze and just sit or stand there not doing anything, if I pause they may come "unstuck," but that doesn't always work, I may have to pause and un-pause several times to get them moving again (and once it didn't work at all and the whole game ended up freezing). It usually happens to two or more of them at a time, particularly if one gets "stuck" while interacting with another, it will affect them both.
I don't have any mods or any custom content in my game, the problem seems to be specifically with either the Parenthood pack or with a recent patch, because it's only started happening within the past few days.