Unstable FPS. I ask for the patch

[Gadavre]

Not sure how such a tool would be created. Optimizing a level is part of the design. If it's poorly optimized, automating such a process would likely cause unexpected side effects.

Do you experience the same issue in the base game? Or only in custom stories?

There are many interesting ways to solve this issue.
1. for instance, the Programmer create a program that can disable all those graphic effects in the mod (custom story) that are not in the original game. 
2. Already there  are programs. which allow you to compress textures, for instance, "Textures Optimizer" for the game Skyrim. Perhaps the Creator of custom history uses  hd textures in his work and tested his work on a very powerful computer. Texture optimization may make such a story available to all players who do not have a super powerful computer.
3. A simple program to unpack the game files can remove friezes and increase fps in large locations. For instance, for a game it might be better to unpack and pack files with a lesser degree of compression than in original game.
using the program, I removed the friezes in the game "State of Decay". But I have not found such a Tool for our favorite game
As a rule, the game engine is much easier to read unpacked files or packed files with a lesser degree of compression.