This is not new information, Toni has posted it on her 'hood thread, but it's such good advice that I thought a summary here would be useful since this is the place where people who are having problems might look.
First of all, the symptoms - which led me to think there was something seriously
wrong with my game:
My sims were no longer behaving themselves. I'd set them a task and it would just disappear from the activities lineup (upper left corner) and the sim would just stand there.
I like to queue up actions. E.g. Use toilet/take a shower/sleep in pajamas. If the whole sequence didn't disappear, often the first action did.
Starting cooking, then not completing the action, no matter how many times I ordered the sim to resume cooking.
Starting to make a 'phone call, then hanging up before even getting started. Very maddening when you're trying to get the repairman to fix your computer!
The sim would quit most actions (studying Mechanical from the bookcase, for example) and just stand there, or in a flash, turn around with a different image.
Now the advice (and believe me, when you need tech advice, Toni's the one to go to)
1) Don't reinstall - there's some good advice here
(Miros in woodensimolean.com)
2) I didn't think this
would fix it, but it did! You will need this
in addition if you don't have it already. It's needed by the No Sim Loaded
hack. You just install these two DBPF packages into your downloads folder (I actually have a subfolder named "Hacks", where I put my... Doh!)
These hacks (and they are hacks) may not solve every problem but they're worth a try. They certainly worked for me!