Two things come to mind as possibilities regarding the "missing textures" suddenly occurring: "too many active plugins" (the nominal range of 130-140 varies by system and mod mix); and problems with the video pipeline. The latter can come about if you have installed many large (greater than 2048x2048 pixels) textures.
Image size matters. A 4096 x 4096 image with 16 bit color resolution is 32 MB of data. If your screen is displaying 100 different models with textures that size, that's 3.2 GB of data just for the textures. Since a 32 bit program can only address up to 4 GB of data in total, you can see how trying to display a lot of high resolution models is going to run the game out of memory very quickly. (Thank you for the perspective madmongo.) Now, VRAM is not directly addressed by the program; it's controlled by the video card and can hide the actual addressing from the program, but you get the idea.
Another thing to consider is that larger images take longer to pass through the video pipeline. When such larger sizes were not considered in the game design (and they weren't at the time), bottlenecks are to be expected.
If that is not sufficient to resolve your problems, Please provide ALL the information requested in the wiki "How to ask for help
" article. Don't forget to identify things that do not appear in the "load order", such as NVSE and it's related plugins, texture replacements (specifically their larger image sizes: 1024x1024, etc.), and "post-processors" like ENB or SweetFX.