I had a problem with a custom world download this afternoon, the details of which are unnecessary but it all came about because I thought there was a problem with it as the terrain and water had gone strange, like permanently stuck in the lowest possible detail (like a lot in the distance that hasn't loaded/rendered properly yet) but I soon found out that all of my worlds were affected.
The lots, objects, water and roads load fine and detailed but the world terrains and water edges are awfully blurred and choppy.
Here's 3 pictures of the custom world and Appaloosa Plains:



I've deleted the caches, I don't have any mods that effect the graphics and the mods I do have are up-to-date and unconflicting.
Does anyone know what this is caused by and how to fix it? I've spent two hours already trying to figure it out D:

Ok, so it looks as though we need to see your system specs. (Especially for the graphics.)

Okie doke. The game's been running fine for about 2 years now btw, I don't remember this ever happening.
I have no idea which part of the DxDiag to put so I'll just attach it, hope that's ok

My money's on it being a graphics hardware issue - you have an 8400M GS - a mobile (i.e. weaker) version of a known-defective 8400 chip (it doesn't handle heat very well), and you've been playing TS3 plus at least one EP (Pets, possibly more?), which increases the strain on the graphics. My guess is this is the first sign of its impending demise.

Ah I see, I play with all EPs except Showtime, Skyrim too.
Yeah heat has always been a problem, as it's a laptop on a table it doesn't get much air beneath.
Do you reckon uninstalling any EPs/lowering graphics to minum would help at all?

Maybe, but if you don't wanto kill your computer, i suggest you stop playing games on it.
The model you have is like DV9xxx prone to die due to video-card burnout, the cooling system can't effectivily manage to cool it, which is proberly related to what HystericalParoxysm wrote.

Well seeing as that's pretty much the purpose of that laptop that's pretty unlikely, my parents certainly aren't going to buy a new one any time soon either.
Is there any other way to remedy this without getting a new pc/upgrade? The graphic problem is kind of bearable for gameplay but I've been planning on doing a story blog for a long time now (featured on the island world in my first screenie) and the graphic problems are a biggie when it comes to screenies.
I know Pets has put a lot of strain on it since it's installation so removing that might help, the animals do tend to cause CTDs in animal-heavy worlds such as Appaloosa Plains anyway.

Really, I'd recommend stopping playing... because if you have a laptop which gets hot + a faulty graphics card which crashes when it gets hot, that's a recipe for an ex-laptop. You could try buying a cooling pad - read reviews first, since some are incredibly effective, and some actually just make the laptop hotter - or, as a stop-gap measure, raising the laptop up with CD cases under the corners, or by putting it on a cooling rack (the cooking utensil, I mean). Uninstalling Pets would also help.

I appreciate why you're reccomending to stop playing but it isn't going to be long term - my parents are going to get a proper gaming pc/laptop when we move later in the year so I just want it to last until then.
Thanks for the suggestions whiterider, I'll look into a cooling pad but definitely do the CD-prop-up. I've tried something similar with coasters but they're too thin, I've never thought of using CDs. ^_^
Thanks to all for the help and telling me the problem! Looks like this is as far as I'm going to get with a solution but it's way better than nothing