X3: Terran Conflict, when you're in a big fleet battle while you simultaneously have several hundred factories scattered across the galaxy, and ships ferrying resources between them, while on max settings.
The game becomes so resource intensive at that point that the game kills the AI temporally so your tower doesn't crash and burn.
100 ships + space flamethrowers = LOL, CYA FPS
The recent patches have lowered the stuttering while in large battles, thankfully.
