Don't know if this is a common mistake, but how do I remove these lines? There are no gaps in the floors etc.
It's the fault of your graphics card. It's probably one of the newer radeons yes? You can't fix it without buying a new graphics card (not, that there is anything wrong with your current one, I hasten to add!) I don't think it happens on any Nvidia cards
Yep. When I upgraded from an 8800 to a 4870 the first thing I noticed was those lines appearing everywhere....
I think it may be from the monitor as well. I get them on egypt, at the edges and im running a GTX285. Although not nearly as severe as your pics.