I am experiencing what you said but without corrupted user file. I mostly play on an Invade and Annex mission server (2 different servers) and this happens during a defense mission when units are spawned in. My fps drops and stays around 5 fps with random low texture flickering. I tried playing without mods (I only have JSRS) and fps still drops. Whilst playing on a KoTH server, it rarely happens but when it does it usually when i'm overlooking at a heli with people getting out.