@Hyerlobos Sunset Valley is actually among the most stable EA worlds overall, much more so than Bridgeport, so it's odd that you've consistently had issues there. But it's not perfect, and maybe you've found a couple of problems that most people avoid.
First, my original run was in Mosquito Cove where every time my Sims would use the prebuilt staircase to the second floor, they would disappear, pop back into the floor they were just on and then take the stairs as normal. I actually had to delete the exterior wall and widen the staircase to fix the issue.
This is the kind of problem players can see when sims are moved with MoveObjects On; that happened to me once too. It could also be a routing issue built into the house, possibly random but also possibly the result of using the constrainfloorelevation false cheat with less care than is necessary.
Second, Leighton Sekemoto is always coming up as unroutable. When I go to check on him, he's stuck underneath his floorboards with the baby just outside on the porch.
This sounds like another poorly made house, maybe something you could fix by deleting and recreating the porch and stairs that lead to it. The problem could also be elsewhere on the property though: something that messes with a sim's routing long before they get to the porch. I will say that when I play in SV, I don't see enough errors with the residents of that house to be noteworthy, but I also play on a short-enough lifespan that Sam ages up quickly. Again, maybe whoever made the house wasn't careful.
Lastly, it's the only town that causes my game to CTD. I have played Bridgeport, Riverview, Midnight Hollow, Twinbrook, and Starlight Shores with no issues. Somehow, Sunset Valley will always crash after a while. It even happened after uninstalling and reinstalling the game. Couldn't be CC or mods since I wasn't using any at the time.
I'm still surprised by this, especially given how many people play in SV without much trouble. But if your sims are consistently running into the few errors that the world does have, I guess it wouldn't be shocking that you'd see unusual results. Playing with ErrorTrap and Overwatch would help, and perhaps some other mods depending on the packs you have installed. Especially if you have Pets, you might see routing issues related to strays—SV is definitely not designed for puppies or kittens, or really for wild horses either. NRaas Register can keep a lid on the stray population.
Other than that, it would be useful to know what was happening on-screen at the time of the crashes, but you'd need to be in a position to test a crashing save to properly troubleshoot. An alternative would be to use EllaCharmed's version of SV—it's even more stable than the default version and has been used extensively for over a decade. You'd need to start a new save to use it, as is the case with any world fixed at that level, but that wouldn't be a problem if you don't currently have an SV save you want to preserve.
@Hyerlobos Error 12 is an out of resources condition, usually but not always about memory. There's no way to allocate more RAM to a 32-bit application like Sims 3, although if your computer is already using most of its memory, freeing up enough to give the game 4 GB to work with can help.
It's worth checking whether your SV saves are in fact causing the game to hit the ~3.7 GB RAM limit, in case the issue is something else. You can put the game in windowed mode and alt-tab to the Task Manager to see.
CAS is more resource-intensive than live play, and since Sims 3 doesn't always release all the memory it initially allocates, spending a lot of time in CAS or frequently switching between it and live mode can be a problem. The same goes for Build and Buy modes to a lesser extent as well as Edit Town.
