Graphical driver error or game engine renders?

I'm having some real issues with the graphical quality within some dungeons. I have been running The Throne of Tyranny. (Is that the right name? The Dwarf one in the Underdark, where the illithid is corrupting the throne.) The textures sometimes take an AGE to load in, to the point that I sometimes cannot see the screen, the bosses health bar or any other thing on the screen. Everything looks like the sixties felt!
This can last the entire boss battle time and can ruin the experience.
The problem happens with large volumes of pepl on the screen and fades away as pepl leave. I thought it might be a custom texture thing, as sometimes the boss lvl loads perfectly. I will stress that I have no mods! I run all my games vanilla...
I was running a GeForce 650 but felt that I needed to upgrade it to a 960. I usually let the GeForce Experience choose the best way to run the graphics for the game.

My specs are;
i5 330 on an ASUS motherboard. 12GB GDDR5 memory.
GeForce 960 4096MB GDDR5
1.01TB primary HDD with all the virtual memory buffering (the game is installed on a secondary 950GB)
1Tb LAN (I live within 1 mile of the ISP's switchboard and we are all fibre optic!)
I can actually remember when the building was a switchboard and a dozen pepl used to work inside. Now it's just computers...

I hope someone can help me get this game to be beautiful again.