Yeah, sounds like you have either a hack screwing things up for you there, or a conflict. As to what kind, I have no idea. If this is a recent issue, I'd suggest you go through your Downloads folder and remove the last batch of hacks/mods you've added to see if that fixes the problem. I'd also run
Paladin's HCDU program to check for any hack conflicts. (Sims 2 -> Programs -> Hack Conflict Detection Utility)
...otherwise, you'll have to resort to the 50/50 method: taking out all of your hacks and putting them back in a few at a time until you find the one causing the problem. Fun, huh?
Also, check to make sure any hacks you have are updated to work for the most recent Expansion Pack you own.