I'm currently in the vale of shadows (The huge temple with the pillars and the lever which opens a big iron door)
The game crashes and says "An Assertion failed in ChVidImage3d.cpp at line 840??

I'm using the mods provided in the "IWD Mod Guide & Install Order"
Any help would be appreciated!!

SOLVED: Well, I re-downloaded the game and installed the mods in a different order and that did the trick.
Don't know what the problem was but this is fine

A wrong mod order can easily cause issues so that is more then likely where the issue was.

What order did you install them in Sebek? I also followed the install order from that thread, and am encountering the exact same problem.

Try This Guide. The OP does mods for the game and he says this order is better.

. That is the guide I used... and the original poster as well.

My apologies. I assumed you'd followed the stickied guide.

I also used This Guide
and had the same problem in Black Wolf Temple - just as you try to go down to level 3, after flicking the switch in the pillar.

I tried a reinstall but no joy. I reinstalled again without the 1PP graphical mods and I was able to continue the game with my existing saves.

So something in that mod (or the specific choices I made on install) conflicts with another mod. For now I'll continue without it as it's not necessary.