It's been an issue which is slowly driving me mad, since my game is becoming literally unplayable.
The game freezes (and it stays that way) when I literally do basically EVERYTHING. Changing skin color in CAS, switching between live/build-buy modes, changing time, anything.
I've been looking for the issue in the downloads folder, but I literally cannot find the mod that could be the issue.

Can you post your computer specs?

How long has this been going on for? All the time, or have you been able to play your game normally, and then it just started to lag/freeze at some point? Have you installed any new EPs lately?

Also, do you play with a lot of downloads, both CC and mods? (amount in MB/GB would be helpful)

Do you play on big lots (anything above 3x3 lots), use more than 6-8 sims per family, use high-poly CC, CC with high-quality/large textures, or anything else that could cause the game to spend resources on thinking time?

Just asking, because things like playing on a low-end computer, using a lot of CC, or any of the things mentioned above can cause a lot of lag and/or the game "freezing" while opening menus, especially CAS, build/buy, changing skins, and things like that.

Could be spec related or game related. First thing you should try, is a driver update for your graphics card. But, ultimately, it's difficult to gauge with so little information you've provided.

Do you have the 4 GB patch?

What are your computer specs?

How much CC do you have?

Have you ensured Ondrive and your antivirus cannot read your sims 2 files?

How long ago did you update drivers?

How long ago did you dust it out?