Used terms and their meaning:
Micro-freezing: when your screen freezes (but only in the game) for 0.1-1 second, and your last action is repeated. For example: if you were moving in a direction, you'll keep moving in that direction. If you were attacking, you'll keep attacking.
FSC - firestorm citadel, levels before Vanaduke boss.
This happens both when I play solo and when I play in a party. This happens almost every time when I get into FSC.

Problem:
After I complete depth 24 and move onto 25, my game is constantly experiencing micro-freezes. If I restart the game, its gone.
I also noticed that sometimes when I get micro freezes in depth 24, its always when there are zombies (slag workers?) around me. After I kill all zombies (there are rolling blocks still working) micro-freezing is gone. In Depth 25 it starts after I meet several zombies, but if after clearing one room it sometimes goes away, after clearing another room I get constant freezing till I restart the game.
In short:
I often experience micro-freezing when completing depth 24 of firestorm citadel and going onwards to depth 25. Even if I just restarted the game and went straight to depth 23 or 24.

How to restart the game without losing progress:
shut down your internet connection (don't ask me how, you have to figure it out yourself) and after that shut down and restart the game. Then restart internet connection. The whole point: when you shut down the game, you should be offline! If you are online and shut down the game, you'll be moved to Haven automatically. If you're offline, you will not be kicked from clockworks.