I've been dealing with this issue for a long time, and it's aggravating as all hell. All. Hell. I have a handful of HD texture mods, but I mostly pass them by in favor of using ENBs. I ran Skyrim Performance Monitor to see what was happening under the hood and memory was never anywhere near capped. It's the same every damn time. Play for awhile (30 mins) getting ~40 FPS, go inside a dungeon, come back out and WHAM, 10 FPS. Can't Alt-Tab because it will just crash the game. All my mods are up to date, cleaned with TES5Edit, Bashed Patched, BOSSed, etc. Currently using Ewi's INI files with mostly stock settings. Using the latest ENB dlls, Skyrim Project Optimization, SSME, etc. Google something along the lines of "Skyrim fps exterior cell" and you'll see it's a ridiculously common issue, but I've yet to find a working solution.