I've run into an issue with high res textures that I don't understand, and I'm hoping someone can provide some insight.

I have a GPU with 4GB RAM. I played for months without problems with everything from the STEP guide installed, as well as other replacers. I recently went through a mod testing and cleanup phase, and now have rare CTDs, except for one significant issue.

I frequently get crashes entering or exiting taverns, and occasionally from entering Windhelm during the day (only Windhelm, only during the day). But if I change the Texture Quality setting in the Skyrim menu from High to Medium, these crashes go away. I can go 6+ hours without a crash, as long as texture quality is set to medium.

Settings:

Are you monitoring your system? I recommend http://skyrim.nexusmods.com/mods/6491 so you can see what is going on with your hardware when these crashes occur.