The lag spikes and a missing ceiling

[Filizitas]

You dont know much about that stuff, since you would instantly see two things you said that dont work together:

not much RAM

and the game loading in textures MID-GAME

Do you know what texture streaming is? Its the mos advanced technique out there that is supposed to keep memory clean and stop game from lagging. But its bound to a PC that actually has the min req RAM.
So if you dont have 8 gb of ram delete your thread, cause thats the min req of the game. Anything else is your problem.