Yes, my Skyrim install is fairly heavily modded, but I use STEP and try to keep it all compatible. This is a new problem that just cropped up.

Basically, I get a smooth 60 FPS 99% of the time. But every so often, while doing nothing it drops to 30 or 40 FPS, if I'm lucky, or it just straight up crashes.

The last three crashes have been

A) talking to Lilith Maiden-bloom outside Whiterun

selling stuff at the Riverwood Trader

C) walking through an enemy-free corridor of Bleak-Falls Barrow.

I know Skyrim is unstable when modded, and I expect crashes in scene transitions and the like, but this is new to me. I'm at a loss. My rig is pretty much a beast.

Intel Core i7-2600 @ 3.4 Ghz

16 GB DDR3 RAM

Geforce GTX 970 4GB

256 GB SSD

1.5 TB Data Drive

2.0 TB Steam Drive = Skyrim Installed

I'm going to star culling mods, but I dont' know, I'm hoping there's some INI edit or tweak I've missed that could help. I've done all the ones STEP recommends.

Some Incompatibilities between mods, most probably.

Impossible to know, if you have dozens of mods installed.

You can post your load order, if someone has any clue about it.

Also if you can decrypt the Papyrus log file, i would start with that. !!!

Make any ini edits? What did STEP recommend to change?
Mod load order?
Clean save? i.e. Have you been uninstalling mods from current save?
Are you freeing up background applications before starting Skyrim?
Are you using SKSE and the memory patch?

Yes I did a clean start with this latest build.

yes to the applications.

Yes to SKSE, the memory patch, stable ugrids to load, and the safety load addons for SKSE,

What is uGrids set at. Default is 5 and it is best to keep it that way

How often do the crashes occur? I myself have a decent set up and highly modded game and I've just dealt with the fact that my game can and will crash randomly ever so often. I can play for hours on end with nothing or start the game...chnage cells and boom.. Crash.
If your crashes are not frequent and do not hold some semblance of a patten I would not worry about it.